Smoothbore Trapdoor Springfield?

Got a question from a friend, which is outside my area of knowledge. Does anyone know about the history of smoothbore, .45-caliber Trapdoor Springfield rifles? This fellow bought what was advertised as a forager conversion, but it clearly is not a forager. Aside form the caliber difference, it doesn’t match the date or serial number patterns for a forager.

This particular gun is dated 1873, and has a serial number in the 120,000 range. It appears to be all arsenal-modified, with proper markings and workmanship. The stock has a cleaning rod slot professionally filled, and it has a .45 caliber smoothbore barrel, with a .45-70 chamber. We know there were .45-70 shot cartridges produced during the 1880s (the owner of this rifle has one), but we aren’t able to find much (or any!) information about arsenal conversions of Trapdoors into this type of fowling setup. I don’t have photos, but may be able to get some. I understand that Bannerman built guns along these lines, but this one really doesn’t feel like a Bannerman conversion.

Does anyone know about possible provenance for these? My depth of knowledge on Trapdoors is pretty shallow, and this particular gun is over my head.