I don't know. I'd say a Colt Gold Cup.45, but the bullet holes are larger than those from a Colt Python, a M-27, and a M-29, all of which shot groups that'd have all holes touching at 25 yards, fairly often. Offhand, mostly with factory ammo.
The .45 SWC bullets left bigger holes, but the .357's may have actually been closer together.
I also had a Colt New Service .45 revolver made in the 1930's that'd group all six bullets in one ragged hole. Unlike many Colts, it shot to the sights, too.