there was a side mount for a 39 i just cant remember the make. in 46 / 47 /48 / sears had marlin d&t the barrel for a scope mount .
Yes, you can tell if your gun was a sears sold Marlin by removing the fill screws. If the threads are in the white the barrel was drilled and tapped afterwards by either Marlin or some gunsmith. If the threads are blued then it was a rifle manufactured for sears specifically. The only 39 A rifles drilled and tapped at the factory prior to being shipped where the ones made for sears.
The years Marlin made them for sears was 54 and either 53 or 55. Can’t remember at the moment. It was only 2 years as I recall.