Wow this post... I mean, I know HondaMan you wrote the book on this stuff, and I don't want to sound over the top but I just have to say I am continually humbled that someone with such deep knowledge on the subject is taking the time to weigh in on my issues. Knowledge bombs dropping all over the place! I'll do my best to hold up my end when I've got this ridiculous level of expertise on the line.
So I've found my magnifier and one of the original mains that is less banged up, looks like it has an "105" on it.
So hooray, I have a non-doggie 750.. or at very least, the mangled remnants of the jets from one 
Clearly I need to be using these jets, hopefully their condition isn't going to have a worse effect than the discrepancy of the opening as you described. I guess I'll need to get myself some good mains from a reputable source. Perhaps I should do that before putting back together but I'll admit I'm keen on seeing the effect of what I've done already.
Just wanted to follow up on the missing filters, because my leads continue to come up dry. Is it a major issue to run the bike without the filters on the float valves? Like, should I absolutely 100% for sure wait until I can find some before reassembling? I'm keen on getting it back together, but I want to do it as right as I can... If it's a "you're going to ruin that bike" situation, I'll take the time or the expense to get it right, but if it's a "meh most people don't even have them", then I'll get to wrenching.
At any rate, I've got more parts to clean, the tank to inspect, and dinner to make, so it will have to wait for a while anyway.
The jets being 'beat up' looking isn't a problem, so long as they have not been drilled out by someone. I have always found the OEM jets to flow better, mostly because the well-hole area is larger. Mikuni jets often demonstrate this trait, too.
The jet sizes changed over the years as Honda re-tuned the engine. The sandcast bikes through K0 series usually came with #120 jets and fouled sparkplugs like crazy. They dropped to #115 both with Service Bulletins and "shop notices" (and Honda reps suggesting it) by the end of the K0 series bikes, though, which helped a little bit. The earliest Old Factory K1 came with #120 at first, but these quickly became #115 when the K1-style HM300 exhaust pipes appeared (these have an extra baffle inside, compared to the old K0 pipes). Even these bikes were REAL hard on sparkplugs (mine got 800-1000 miles per set), prompting Honda to tell riders to "ride more spiritedly" in Service Bulletins to try to keep them cleaner. We had lots of lead in the gas in those days, and the D8ES sparkplugs could not clean it off, so the lead fouled them, usually in 1500 miles or so. It was a big problem!
During the early K1 New Factory bikes, Honda started using the #110 mainjets, all else being equal (#675A carbs, New Factory production), while the Old Factory bikes still got #115. While this led to the legend of "old factory bikes are faster" (some of which was true...), we still solved the plug fouling in 2 way: first, we installed #110 jets and then we drilled out the emulsifier tubes to 0.037" holes in the upper holes (which is the lower RPM ranges) to aerate a little better. This helped a lot. One of the 'unknown mysteries' about the #657A carbs is: they have brass floats that are staggered in height by 1mm. The higher one is set at 25mm from the tiny inner sliver reference post, so the other float is at 24mm. But, if you measure instead from the notch in the side of the carb body (not the inner sliver surface), you must use the more-commonly quoted 26mm height value, as those outer notches are about 1mm taller. This causes lots of confusion...
In the 657B series carbs, the idle air screw changed (lost the expensive holey tip) and the floats went to plastic. They were still staggered and adjusted as described above, but when the new-style float bowl gasket is installed, the inner sliver is buried, so the floats were referenced in the manuals to be just 26mm height from the outer notches of the carb body. These carbs started out as #110 mainjets with the new HM341 pipes and milder cam, and slower spark advance curve, all to make for a more gentle start from a stop (the early 750 starts like an in-out box instead of a smooth slip of the clutch). By about 3/72 production, the mainjets dropped to #105 size and the floats became non-staggered in adjustment, which really reduced the plug fouling issues a lot. It also dropped off a couple of HP at top speed, but the US was suffering the new 55 MPH speed limit in those days, so not many people noticed.
About this time, ND came out with the X24ES-U "Hot U" sparkplug, designed to be 1/2 step hotter (7.5) than the D8ES from NGK. At the same time, NGK was asked by Honda to help with the plug problem, and they answered with their D8ES-L plug, which is about a heatrange 7.6. The ND plug has a thinner tip, which lets more airflow around the spark, compared to the D8ES-L, so it has a slight edge in ignition performance. It's slightly hotter tip also helps keep things cleaner, by a hair, than the D8ES-L. I use these ND plugs exclusively, after 2 years of side-by-side testing done by my shop in 1973. They make a tad more midrange power and stay cleaner much longer than the NGK versions.
It seems the ideal mainjet for the engines made after 1973 turns out to be the #107.5, hard to find. It is often marked "108" for space reasons. The 105 makes the top end a bit flat at our 75-85 MPH Interstate speeds today. If you live at low altitude (below 3000 feet) you may be able to get away with the #110 mainjet, if you use midgrade gas instead of premium when riding in town. The ethanol in today's gas helps to clean off the plugs, even as it strips all the lube off the valve stems and top piston ring (but that's another topic!).
When you change cams or airbox inlets, the jet size usually needs to be modified. Sticking on 4-1 pipes of the kind we see for sale today won't change tuning, because they are so short they don't extract across the piston during cam overlap. If you get longer pipes (like the Sito 4-2 from Italy), then, yeah, you might need to jet up a little bit: I recently discovered this on a K5 I just finished resto-modding for someone. These pipes act like the old Dunstall headers, which were about as long (clear to the back of the rear wheel!), and add some noticeable top-end power.
