Bubba-7

It isn't known for sure where the S marked slides came from. One guess is the A. J. Savage Company, but there is no proof of that. Savage, among other manufacturers, had a contract to manufacture the Model 1911 pistol, but they did not get into production before WWI ended and the contract was cancelled with no pistols being delivered. It is known that a few small parts were delivered to Ordnance after the contract was cancelled, but no slides are mentioned in these parts. It was a requirement that the slides on the Model 1911 pistols have the manufacturer's name, which was an Ordnance specification, and the slides in question do not.
The patent data on the slides is identical to the Remington-UMC slides, without the Remington logo and name. Sedgley did do contract work for Ordnance, and their logo was a circled S. Possibly Sedgley bought left over slides from Remington-UMC and removed the logo and address, and added their circle S.
The required finish for the Model 1911 was blue, and phosphate was not used on new production until 1941 on the Model 1911A1. The slides in question were used in rebuilds, and a 500K serial number Model 1911 Colt could only have gotten the slide in a post WWI rebuild. In all likelihood the finish in rebuild would have been phosphate.