I've had two OMM - - -
- - - And my son has an Officers Model Special. All have had the lettering on the barrel pretty well centered between where barrel screws into frame and the muzzle. If YOUR markings are well toward muzzle end, I'd bet good money the barrel has been shortened.
FWIW, Colt offered the Trooper model in both .22 and .38 Spl, in four and six inch, to begin with. Later, they rolled the "Colt .357" model production into the Trooper series and all were so marked. I assume your four-inch IS marked OMM on the barrel, right? Because if it isn't, possibility exists that someone put a Trooper .22 barrel on your frame, having removed the Trooper markings.
Another, smaller but still distinctly possible, alternative, is that you have a special-order gun there. Long before the advent of the Colt Custom Shop, they would provide special-order stuff at a very nominal price. IF yours is marked "Officers Model Match," with caliber, and the markings are centered, it would surely be worth some research. Colt factory letters are getting pretty expensive, but they will often answer questions by phone, to tell you whether or not it would be worthwhile to pay for a letter.
Suggest you go to
http://63.241.174.172/colt/html/i2b2_researchablemod.html
and branch out from there.
Let us know what happens, huh?
Best,
Johnny Guest