Sigh it is worse than that. Here are better lighting pictures of the one I showed. Seems it isn't a re-blue but instead a corrosion secial. Mechanically the gun is good. 4 clicks, tight lock up, etc. What we seem to have is a gun wrapped in a damp rag then ignored for a really, really long time. Long enough for there to be nothing but pitting where any writing should be. Yes of course the base pin indicates it is black powder only. That is a given.
The original caliber is utterly unknown at this time. I see no option to the letter if I want to know what it was originally. It is not ..45 Colt (they won't fit). It is not .44-40. That doesn't fit either. Naturally .44 Special doesn't fit either. Too long. How could it be that anyway? The gun predates that cartridge. However, a .442 Webley load drops in as pretty as you please. That's a problem. Colt never made a .442 Webley SAA. BUT they did make a few in .44 Russian which is just an oversized .442 Webley anyway. They also made some other strange .44s like .44 German, etc.
But I still got a problem if it is in .44 Russian. That is the serial number is too early for that cartridge (according to Kuhnausen anyway).
The serial number of the frame is pitted away, utterly. The trigger guard has only 3 readable numbers I can just make out a ...603. On the butt however are two lines of numbers. Line 1 is (barely readable) 68. Line 3 has 603. So #68603. That equates to being made in 1881. Way before Colt made SAAs in .44 Russian.
Yes, I know the perils of assuming a removable butt serial number is the same as the frame serial number, just because the last three digits match those of the trigger guard. But there is no other option.
So my thought is get a Colt letter for 68603 and learn the caliber. Maybe learn it was shipped to some kid name of Theodore Roosevelt in 1881. That'd be nice.
What do I do if I get back a letter saying .44 Henry CF or something like that? Keep the pitted cylinder and barrel, or swap them out? Put a new barrel and cylinder on a pitted frame with not even a patent date visible?
For right now I will just use (black powder) .442 Webley ammo in it while pondering the next step.