Springed riveted bracelets (the ones Yuki sells) are available in several mid and late 70's catalogues without an official year of withdrawal. I have a few of them, some show riveted bands on "non officially riveted" models, some don't.
On some models the serials were even reset, creating a messy situation with old papers, and thats why imo Rolex is not officially servicing old models anymore. It's not that they can't, it is cos they aren't able to positively path every serials on vintage models and are scared of good frankens.
Provided his cosmograph is a 6265, officially on 6265 there are two options:
1) ref. 7835 with 271 or 371 endlink until 1974-1975
2) ref. 78350 with 571 endlink.
Well, I have a few mid and late 70's catalogues with 62xx with rivet bracelets, even gold ones ^^
Take my word, everything is possible. My gen 6241 should have a 7835 according to its age. It came instead with a 6635 (which btw the previous idiotic owner had broken into pieces, curse him), probably because that's what the buyer ordered or simply found in that shop. I have seen 78360 and even 7836 Jubs on daytonas and SDs, which is not officially possible and somewhat weird.
Some ExplorerIIs (1655 on USA market) came out with a 7206 (prolly custom orders, officially 20mm riveted 7206 only on GMTs 6542-1675 and Subs 1680). I have seen tons of 551x Subs and 1665 SDs too with Jubs on em. There is not a single model NEVER mounted on this or that model. The only serious limit is lugwidth, (which is the issue we are discussing here).
Rem that Rolexes were never seriously back ordered until late 90's, riveted and springed bracelets had half-heartedly success until they became vintage, so anything is possible on custom orders.
Look here, 1978 6241 with riveted springed bracelet (officially discontinued around 1970 as you said, but that's clearly a lie)
http://www.mazzonishop.com/scheda_articolo.asp?IDARTICOLO=590
Anyway, if AceofDiamonds was misled by my link, I will happily buy his 20mm Yuki's bracelet full price + shipping, so he can fully refund his wrong purchase and search a proper 19mm one, I will have something I need and we'll all be happy