Tinker,
Yeah, I agree.
Plenty of evidence that it was originally a shottie.
Wide bore spacing at the muzzels, wide rib, top rib joined where it meets the quarter rib, large diameter case head marks on breechface, sleeves showing on barrel face, non original extractor, crown over "w" indicating choked barrel, etc etc.

I do think there is a chance that it was originally a 20 gauge.

Note the ridiculous position of the rear scope mount lever which appears as though it could prevent the gun from closing.