it says in my Illustrated Directory of Guns (p.230 top), and referring to my gun, that "This .38 Special w/ a 4" barrel was made specially for the Railway Express Agency."
there's another pic on the bottom of p.229 showing what looks like the same gun, only where mine (p.230) says (read pic below) on the barrel, the other one (p.229) reads: Police Positive .38 Special.
so is mine actually a .38 special, or just a .38? the text in that article says that this Colt also came in a .38 Model. but when looking at the text referring to my gun, it says it's a .38 Special for the Railway Express Agency...
sorry for the confusion