Sounds like you have a 2.5-inch bbl. Diamondback. (They may have made a few three-inchers, but they'd be rare, and I don't recall that they were catalogued, but Colt has done some strange and wonderful things.) If the sights adjust as they SHOULD (something that once gave me trouble on a Diamondback), you can shoot any common .38 Special round in it, but Colt advised that the company should inspect it if more than 3,000 rounds of Plus P had been fired. (For lightweight alloy frames like on their Cobra or Agent, they suggest checking after 1,000 rounds). Just don't shoot Plus P in casual practice, and it should last well.
Because Colts of that model and era are becoming collectibles, and because this one is a family heirloom, I wouldn't shoot it much, except for occasional nostalgic recreational firing.
I'd add a Tyler T-Grip adaptor if the grips are too small, or get a set of older Pachmayr Presentation Compac grips of the two-panel style at a gun show... if you can find any. The supply seems to be drying up.
What on Earth is the matter with people today??? Never thought you'd own a revolver! Good Lord, man! They're hardly obsolete, and are probably the best handguns made for most purposes.
Better not handle this one too much, or you'll realize how wonderful they are, and want more.
Oh: yes, Break-Free CLP is OK, but another post around here somewhere says that they give funds to the IACP, which is anti-gun. So you may want to substitute Rem-Oil. I clean first with solvent like Hoppe's famed No. 9, then wipe off the gun lightly with oil. Don't overdo it, especially in the bore and chambers. It's not only messy; it can raise pressures. Actually, it sounds as if you ought to begin reading gun magazines and basic books if you don't yet know how to clean a gun. Not trying to be cruel; I just have the impression that you're very new to firearms, and it's essential to learn some basics before you ever fire a gun.
Lone Star