There are/were actually 3 different versions of the HM341, at least up to 1975 when I quit keeping track of them. The first were like yours, we called them the "3-baffle" pipes. They had the best performance of the HM341, and that's what my very early K2 had when new.
Soon after that, they came out with the ones we called "7-baffle" pipes. These had a shorter pipe into the muffler and it did not have the diffuser holes in it like yours does. The pipe ended about 2" in, and there were 7 visible sets of spot-welded baffles on the muffler. These rusted out so fast that Honda stopped making them in about 4 months, and many were warranteed-away so Honda would not "lose face" over it. They rotted right at the upsweep where the pipe joined the muffler, and at the next 2 baffle spot welds on the insides (i.e., bike side) of the pipes. They were VERY quiet, while they lasted!
Then came the ones that seem to have remained the longest, and are being replicated in the ones we've seen recently from a cloner somewhere, maybe in Thailand? These were called the "5-baffle" pipes, and they seem to be in 2 variations: the earlier ones have no inner diffuser on the end of the pipe, but the pie sticks in about 4" inside the muffler. These have a throaty note that comes from the drain hole that is punched from the inside-out at the upsweep bend on the mufflers. These have 5 visible sets of spot welds along the pipes to hold the baffles, but I have not seen one apart, yet. the other variation has a diffuser end on the pipe, much like the one you show here, and 4 baffles that can be seen as spot welds along the mufflers. These first appeared on the later K3 bikes and were most of them until I quit tracking in 1975. They last a long time, are not too restrictive (due to the reverse-flow design) and only the #2 pipe tends to plug up its too-small drain hole and rot at the upsweep site. The chain lube usually clogs these holes shut, on the left pipes (a good place to keep an eye on!). These made the 55 MPH touring more practical during that era, because they both boosted the midrange torque (3800-6500 RPM range) and let Honda install a #105 mainjet to 'help' everyone save gas when we were suffering the Arab Oil Embargo of the later 1970s.
I had the 4-baffle, 5-chamber pipes on my 750 until 3.5 years ago when I bought a set of the clones. These are lighter metal, lesser chrome, and a little louder overall than the Honda version, and need a slightly bigger mainjet (like 2.5 size bigger than whatever you had before) to cool off the engine a little at hiway speeds. This hints that they don't have a good reverse-flow pattern inside, so there is more header effect 'sucking' on the exhaust valve at speed.
