Saw cut receiver, Complete but has 1/4" gap between all cuts.  Clean but obviouly well used Marlin Rockwell M1918 Modified to A2 configurationl.

Marked 'Browning Machine Rifle' and Serial number 36943 so it is from the first contract run of Marlin Rockwells.  The receiver is also stamped:

Crossed Cannons stamp

A2 stamp (after US MODEL OF 1918)

RA for Raritan Arsenal

OG for Ogden Arsenal

the Rear seems to be from an IBM based on the selector font, so it is probably Armasteel