Oh, man, do I feel dumb! Thanks, flitecontrol
, for the quick reply. My first reaction was to think, "yeah, I know that but there's no place to put it". But I had been looking around and finally found a photo
of the part here on Partzilla: https://www.partzilla.com/product/honda/16165-KBG-770
I measured mine and found it was 1/8" longer. Then I realized that it also had a longer section beyond the left edge of the part in the photo. My main jet was there all along, screwed in place!!! The angle's a little off in the second photo above, but the light oxidation on the part made the two of them look absolutely like one seamless piece. I guess I should have wondered why they put flats for a wrench and a slot for a screwdriver on the same part.
A tight grip with a wrench and a solid twist of a good-fitting screwdriver, and poof - two parts! I'll be shaking my head over that for a long time.....
The original part I just discovered/removed is labeled with a little symbol and "108". The new part has no markings. It appears that the hole in the new part is slightly larger, but the old one might be narrowed from corrosion, etc. Does swapping the "108" for something a little larger sound like a good idea? I definitely belong to the "it always needs a little choke or it will stall" club, so running a little richer is appealing.