I once owned one of these in this same serial range with no sling base in the wood of the butt - but it has been converted to cartridge, and so had a replaced buttstock as part of that conversion. It did not have the hole in receiver for sling bar. At that time, I had found some reference to naval carbines being converted sort of by accident, but then rejected for the post-war cavalry, because they DID NOT HAVE THE SLING BAR needed for cavalry use and it could not be added because the naval carbine receiver was never drilled for installation of such a sling bar. There was some sort of report I found that naval carbines (used on the Mississippi by the "brown water navy") and cavalry carbines (used by cavalry troopers on land along the Mississippi) were turned in at end of war for storage, and when the armory was asked to supply carbines for conversion, they sent some naval model ones along, and no one noticed until some were converted to .50-70 cartridge.