My Dad had one of the original Sentinals. 6" barrel looked just like a .38 Special until you saw the holes. All steel, I think it had wood grips, don't remember any more. Made by High Standard for High Standard.
Great gun, DA was heavy SA was decent. .22LR (not magnum).
Later versions were made with aluminum cylinders (and I think steel inserts) have seen them at gun shows in the past. Aways passed on them.
Don't know about the MK IV guns, sorry.