The gun is a 20 ga Army&Navy. I thought that the 16.3 and 16.4 marks would represent the bore diam. in mm and would be the newest proofs along with the "20X70" over "950 bar". THe barrels measure .636 RB and .639 LB which is less than the 16.3mm=0.641" and 16.4mm=0.645".
The marks on the barrels in front of the flats are "20B" and "not for ball" and "M" on both barrels.