The Colt NM Mk III may look like a standard 1911, but internally it is very different. It is a blowback operated pistol, it does not lock up using the Browning system.
I don't know of anyone making magazines for this pistol today. Further, the slide lock tab on the float of the original magazines is rather frail.
Phil Stewart, a dealer in magazines, usually has a number of original magazines in a range of conditions varying from pristine to repaired. The magazines are scarcer than hens teeth, and the prices asked reflect this fact.
Phil Stewart
1007 Howard St.
Mt. Vernon, Ohio 43050
His phone number is (740) 393 0704, though he can be very difficult to catch.
Tell Phil that I gave you his name.
Bob Foster