I know of somebody with a late 1960s TC Contender. It was purchased with .357Mag and .22WMR barrels, so .357 is definitely GTG! They've also got a .222 Rem (purchased new with the gun),and a .410/.45LC. All of that being said, I'd go for a .357/.44, the frames can handle it, they're easy to reload for, and EXTREMELY accurate, and I wouldn't hesitate to take a shot (at a reasonable distance) with a .44Mag!
As for the split lock/one piece lock, I'd have to go dig mine out and check, but I "think" that all of mine use the single piece. If I remember right, I did quite a bit of research and found that the split lugs came along as TC started doing the Easy Open conversions on the G1 Contender frames (NOT advised to send it in).