You're not wrong. The eyes and eyebrows look a lot like Christopher Lee. Since in my head canon the gangster in Diamonds is the same one who later visits Scaramanga's island and gets shot dead it is funny that he later dies right next to the mannequin of the man he once drove to the Las Vegas funeral parlor.
I never took those two as the same character. The one in Diamonds is just a general-purpose flunky. The one in TMWTGG seems to be a hitman of reputation to have been worthy practice for Scaramanga.
In my head canon the gangster from Diamonds did something really important between 1971 and 1974 to be entrusted with a visit to Francisco Scaramanga to execute a business transaction. No pun intended.
I just don't think that there's much stock in assuming that two characters played by the same actor are the same character, in a franchise so rife with recastings of the same characters, often with completely different types.
I wouldn't even have run with it had other Bond enthusiasts and Internet content providers not hooked onto the same thing and conjectured it's possible. It certainly doesn't help that Marc Lawrence's outfits in both films are practically identical and he plays a Mob thug in both movies. It's a popular theory in some Bond fan circles and one of the few of the common Bond fan theories I entertain.
The Marc Lawrence Theory is one you can take or leave. But if you seriously believe the Code Name Theory you just haven't been paying attention.
Also Robert Brown, if we're not assuming that Admiral Hargreaves became M. Shane Rimmer had uncredited roles in YOLT and DAF, and played the American sub captain in TSWLM. Walter Gotell's SPECTRE agent in FRWL may or may not have survived.
We almost got Grace Jones in the most recent Bond film based on rumors she had been approached to appear and things fell apart. That would have made her the first supporting actress after Maud Adams to appear in the franchise as two different female characters.
Without getting specific in a non-spoiler discussion, there are more than a few OHMSS references in No Time to Die.
If we're going as deep as extras, there was one guy who appeared in a solid string of the '70s and '80s films...he was either in the production staff or connected with it...who was always used in public reaction scenes to outlandish Bond stunts. IIRC, the first was in TSWLM when the Lotus drives out of the water onto the beach. Then the gondola scene in Moonraker, and when they skied over the tables in FYEO...not sure offhand what the others were. He was sort of the Stan Lee of the Bond franchise for a while there. I'd be shocked if nobody here knew the guy's name offhand.
Including two DS9 appearances, one of which was as, you've guessed it, a gangster in Badda-Bing-Badda-Bang Other multiple film/character appearances. Ed Bishop is a NASA technitian in YOLT and then goes on to play Klaus Hergersheimer in DAF My own personal favourite though is Tsai Chin, the woman who "betrays" Bond at the start of YOLT reappears almost 40 years later as a poker player in Casino Royale. That's got to be the longest gap between appearances in the franchise. For most appearances though, Michael G Wilson?
Only two Bond girls have appeared in more than one film over the stretch of the franchise and as the exact same character. Sylvia Trench in Dr. No and From Russia With Love and over half a century later Madeleine Swann in Spectre and No Time to Die. Tracy is, of course, directly referenced or alluded to on multiple occasions after 1969 and Vesper is frequently spoken of and referenced throughout the four films set after her death.
If Barbara Bach hadn't declined we'd have had XXX briefly in AVTAK I don't think there's any reference to Vesper in Skyfall.
Pola Ivanova was a weak replacement for Anya Amasova. Seeing her and Bond reunited in the Zorin investigation would have been one of the most fun moments in the entire franchise.
Rigg would definitely have been the Bond actor from the '60s who'd have gotten an Oscar were in the position to give them one. She elevated an already terrific script to the next level. OHMSS would likely have been great with another actress cast as Tracy but with Diana Rigg it became something even higher.
There's barely a handful of performances from any of the Bond movies that approach the level of deserving an Oscar nomination. Rigg in OHMSS is one, Daniel Craig and Eva Green in Casino Royale are the others. Maybe one for Judi Dench for Skyfall, but that seems like a sentimental nomination as much as anything.