It's a mystery!
The SN format on the grip frame is that of a commercial or .455 N frame of the era, but the inspector's mark in the yoke cutout as mentioned above indicates it was a leftover 1917 Army frame. All well and good, as 1917 Commercials were built with this SN orientation...but matching numbers in the half-lug and cylinder? That strongly suggests either a factory rebuild/conversion or a really obsessed gunsmith.
A letter of authenticity would be a good idea, I think.