I can only guess that this is a factory 1st Model frame that was leftover from the first model that had the recoil shield machined off to be used in the 2nd Model S/S series.
MT99 - Is your gun frame machined for the cylinder hand and cylinder frame? Some first models with the modified cylinder shields have them, have one machined, or have none machined.
The hand and cylinder stop can be a little puzzling.