The only place where there are issues:
1. The 1975 "K" harness is the same as the 1976 "K" harness until the last 3 months of "K" production, when it got the "F" harness. The 1976 "F" harness changed by separating some wires out of the headlight and sticking them in a little plastic box under the triple tree. So, some of the wires were a little short to reach inside the 1975 headlight as the result. Extending those 3 or 4 wires about 2" apiece is enough to fix it up. A few rare late 1976 "K6" bikes (actually retitled leftover K5s, with different decals, sold in 1976) have little extension wires that do just this, and they do not color-match the originals, but have little white bands on them near their end connectors. These were adapter wires. Most of these that I remember working on were Planet Blue in color, too.
2. The 1976 harness changes in the headlight and keyswitch areas. While the wires did not change colors, they came fitted with 4, 5, and 6-position connectors instead of lots of little bullet connectors. The aftermarket harness you are looking at will work fine with these, but you have to change some connectors to get it all plugged together. Also, there are a couple of wires to the fuseblock (middle fuse) that are a different color in the 1976 and later harnesses, as compared to the ones in that aftermarket harness. You can either bypass that fuse and plug the headlight directly into the BROWN at the hedlight shell, or else figure out which 2 wires go back to that middle fuse for the headlight circuit. Either way works fine.
As an example:
I just fixed up a 1976 K5 that had the right handlebar switch from a 1976 "F" and the left one from a K4. The wires and colors are all there, but if plugged directly into the matching colors for the K4 switch, the headlight will not come on when the Start switch is released (i.e., when it should). So, the Black/Red wire from the Start switch then goes into one of the 2 Brown/White wires that go back to the middle fuse, and the other Brown-White return wire goes into the left handlebar Hi/Lo switch. Sometimes, depending on how far into Frankenbiking the PO got, the switch on the left might have a Brown/Red feeder to the Hi/Lo switch instead of a Brown/White: the later "K" bikes (K7/8) sometimes had these, but the switches fit the earlier bikes, too.
The key: if you install a new harness, the most likey thing to show up is the lack of a headlight if your bike has the K3 or later Start switch on the right side. These came in 3 flavors:
a. In the K3 the Start switch had 2 wires on it: Yellow/Red and Green/Red. The Green/Red "grounded" through the Safety Module if the clutch was pulled in or the bike was in Neutral, which then grounded the Yellow/Red wire to the low side of the Start solenoid when the button was pressed. This version does not kill the headlight when starting, but instead has an ON/OFF headlight switch on the right.
b. In the (late) K4 the Start switch got 3 wires. These were Black, Yellow/Red, and [either] Black/Yellow or Black/Red (depending on which Honda factory made the bike). The Black is Ignition power, the Yellow/Red is [again] the Start solenoid, the last one is the "switched headlight" that goes Off when the Start button is pressed. This circuit puts Ignition power into the Start switch, and when the Switch is relaxed it connects that power to the Black/Red (or Black/Yellow) wire, which then connects in the headlight bucket to go back to the middle fuse, then back again to connect to the Lo/Hi switch on the left handlebar (which then actually returns to the headlight, whew...). Most of these bikes had the matching Black/[whatever] color going back to that fuse, but it changed to Brown/White before it got there, just to confuse folks, in many harnesses. It then came back to the headlight bucket as Brown/White to go into the Lo/Hi switch (NOT into the multiport Brown/White connector there: that's the tailights). When this Start button is pressed, it puts power to the Start solenoid on the Yellow/Red wire, and these Start solenoids have one wire are Yellow-Red and the other as Green/Red (which goes to the Safety Module or to a diode, to get to Ground).
c. The bastardized versions: starting in the K5, Honda seemed to use whatever combination of switches and fuseblocks and body harnesses they had that day, literally. The headlight's electrical circuit was the same as the earlier K4 bikes, but since the published wiring harnesses often did not match [even] the customer's bike, it got challenging to alter things for fairings and the like. In these bikes, the [formerly] Black/Red (or Black/Yellow) wire might be found to be just Black, or Black/Brown, or even just Brown, coming from the right handlebar. There are often short Black wire jumpers found in these headlight buckets, with little bands around one or both ends of the jumper to [sort of] indicate which way they were supposed to go. This is where most of the confusion seems to reign. In most of the very late "K" bikes, though, the middle fuse of the block has 2 Brown/White wires to that fuse, so at least it is easy to hook up, as it makes no difference which Brown/White is used.
To make matters worse, the 75/80w halogen headlight appeared about this time (1975) in the right physical size, and many Start switches got melted internally from trying to source too much power to those beams. The only solution there is to remove the headlight, or use a relay and let the Start switch toggle the relay for the headlight power.
Then came the "new keyswitch", up on the instruments. This one usually combines the Brown/White circuit and the Brown circuit inside the white connector on the back of this switch, or in some cases the switch itself replicates this connection via the internal contacts. This means that if you have a bike with the Brown/White tied to the Brown in the plug, you can use any version of the switch. But, if you have the one where the switch does the connecting (typically the 1976 F0 had these) and you use a later model switch ($18 from PartsNmore), you have to jumper the Brown/White and the Brown yourself. This can be done either inside the headlight or with a blue 3M splice behind the headlight shell where both wires are near each other.
If you're confused and have a mixed bag of parts: unplug your headlight and get a $1.99 multimeter from Harbor Freight. Connect the meter to the Green and White wires of the headlight plug, turn the Hi/Lo headlight switch on Lo, and key ON, Kill OFF, and start plugging and unplugging the colors mentioned above. Soon the meter will show 11.5 volts or so (which means the headlight will work), and you'll feel like a genius again. Then you can use the ohmmeter part of that meter to figure out which wires go to your middle fuse.
