It is a .38 Military and Police, square butt variation.
I'm having a bit of trouble making out the serial number. It appears to be 3280 plus something. That, and the stocks, put it in about 1919. It possibly shipped in 1920. It has the optional 6" barrel.
As you pointed out, it went back to the Service Department at some point - star on the butt and barrel. Under the left stock panel you should find a date stamp (like xx.xx). That's when the service was accomplished.
Nice older piece.