If those barrels were replaced, they must have been replaced at the gunworks; but why do you think they are replaced? The stamp "HAC" stands for Hunter Arms Company, and "Nitro Steel" became the standard barrel type for the Pigeon grade (what you have described is Hunter's Nitro Steel Trademark). And yes, Hunter would make all sorts of special stock configurations on special order to include raised combs, Monte Carlo combs, and even cheek pieces. If the gun originally had a pad, it was likely a Silvers: Pachmayer pads were introduced in the 1950s. I doubt the barrels are cracked; more likely just stratched, but that is a situation for which you should consult a skilled gunsmith (barrel specialist).