re: Was John Mason (the Rock) actually James Bond? Interesting similarities

I could definitely buy it, and truth be told, it fits perfectly if you want to maintain a coherent timeline.

As pointed out, Mason's capture and incarceration fits perfectly into the period where Connery leaves and Roger Moore steps in.

Various nefarious villains know of Bond by name, but not by face. Hence, the iconic introduction, where he casually drops the "Bond, James Bond" on them.
So British intelligence keeps the lead 00 guy as "Bond", carrying on the aura of invincibility- and also maintains deniability for the guy in US custody. "Bond" is on a mission to wherever, so you OBVIOUSLY don't have him locked up.

This might also explain why "Bond" continuously gets reintroduced to "Felix Leiter", the CIA operative he works with. Same type of codename, used to indicate to "Bond" that this guy isn't a rookie, he's on the same tier. And again, a different guy in a lot of the movies. Seems like as many Leiters as there are Bonds.

As for why he'd be kept in lockdown, it would be clear Leiter wouldn't reveal who he is unless requested; and it would be equally clear that whoever this dude was, he was "elite and important".