Marlin continued to service LC Smith shotguns after they purchased the company. I once owned a new set of JM stamped LC Smith barrels for a regular framed gun exactly as you described with no markings on the top of the barrels. There's no record of Marlin having produced guns on the regular frame, only on FW frames; so this is a clear indication that the in-house service department was continuing to repair earlier guns after the Marlin acquisition. Most likely the beavertail forearm was ordered along with the barrel replacement, if original; but many Smith guns have had beavertail forearms added after-market. An examination of the fore iron usually reveals factory original vs. after-market. It is highly unlikely that your barrel replacement is noted in the original shipping records, but there are a few exceptions; so I encourage you to order a research letter. A research letter will make your gun more valuable regardless of whether your repair was recorded.