weekendrider, thanks for the response and the links. I did a lot of test searches on yamahapartsandaccessories, and unfortunately they have a lot of the same omissions as on Partzilla or boats.net. For example, here are the available diagrams on Partzilla for a 1976 XS650C:
http://www.partzilla.com/parts/search/Yamaha/Motorcycle/1976/XS650C/parts.html
Where is the carburetor, front wheel, rear wheel, etc., etc.? I know from experience to back up to the 1975 XS650B diagram list in Partzilla to find those components, but what if it's a model that I am not so familiar with?
Anyways, so look at the parts diagram list for the same 1976 XS650C from yamahapartsandaccessories (sorry, their HTML doesn't allow me to post a link), and yes, they show the carburetor, but still no front or rear wheel.
Here's another example, the headlight drawing and parts list for a 1976 IT400C from yamahapartsandaccessories:
https://www.yamahapartsandaccessori...ts/street-mcy#/Yamaha/IT400C_-_1976/HEAD_LAMP
Where is diagram part #3 in the parts list?
I also tried plugging part #s into the search box at the top of yamahapartsandacessories, and the #s aren't very well cross-referenced with their own parts diagrams. Results are a crap shoot. Partzilla does a much better job if you know the part number, as long as that number is in Partzilla's system, which is another crap shoot.
Your second link, zedder, might be great, and I'd pay for it, except that it doesn't have diagrams.
And the third link you gave looks useful, but only covers a fraction of Yamaha models.
Thanks again. I do think that partsandaccessories was the "factory" link that I referenced in my initial post, but it still has lots of omissions. My guess now is that a really perfect reference doesn't exist.