The only 8 3/8-inch heavy barrels made were to be used in making 44 and 41 Magnums. I suppose this is the source of the barrels for the long barreled 25-2s. I don't know what the forcing cone looks like when the barrel is forged and before being fitted to a frame. I know the length of the barrel on the 25-2 measured from the frame is 7 3/8-inches, whereas, a 44 Magnum barrel measures 7 1/2-inches. The ejector rod on the 25-2 is also about 1/8-inch shorter as is the ejector rod shroud.
Bill
I don't know how long the barrel shank on the forging is either, but I'd say that explains how the OP's barrel was made, all right. There are only two basic lengths of protrusion into the cylinder window by the breech end of the barrel on the N frame. One is the length of the 41/44/later 45 Colts, and the other shared by the 25/27/28, etc. The difference is about an 1/8". It sounds like a 41/44 blank was machined down to allow enough protrusion to get to the cylinder face. The barrel marking sure looks factory. The model marking on the frame is consistent with the two 25-2s I have from the early '70s.
USMC, I imagine the rifling is the M25-style 6 narrow lands/wide grooves as opposed to the more typical S&W 5 lands/grooves?