So it would appear it does have the horizontal vent carbs, and the numbers stamped on case halves by sprocket say 975 on both sides, what does that mean?
Hi Chip, sorry for the delayed reply. Ive been curious about the numbers (sequence numbers) you posted a pic of for a while and thought of them as a better indicator of when the engines were assembled. For reference, my friends engine #173 has a sequence number in the 8xx range (VIN #540). Another engine #17x I have pics of has a sequence number in the 2xx range. So the engines were likely produced, tested and later assigned an engine number. Eng #173 was likely produced around the same time as your engine, but assigned an earlier engine number, one from an engine which perhaps didnt pass inspection. VIN 540/173 was built with engine and frame parts in the VIN 5xx range though (no horizontal vent carbs). Your engine however is a later engine number, but given the horizontal vent carbs were only used on the first few hundred bikes, It may be that the engine originally assigned to your frame at the factory didnt pass inspection and Honda transferred the earlier parts to a later engine and then installed that into your early frame.
It would be interesting to know also if your engine has the early waffle cover too, which might suggest that Honda transferred all the parts from an earlier engine to your cases.
In any case, no reason to doubt the pairing. There's been quite a few instances of larger VIN/Engine number gaps on early bikes and when you take into consideration the sequence number rather than the actual engine number, it brings a little more clarity. Thanks for replying.