To my untrained eye, it looks to me as if the end mill has been used to remove previous markings of some sort. I know I'm probably wrong and it's just my suspicious mind working overtime but we used to something identical with out No2 Brownings and L9 pistols. Replacement slides and barrels would come from Ordnance as P/W or Part Worn. We'd mill out the small recess and over engrave the 2T1234 or BL68A1234 to give the P/W slide and barrel a new lease of life.

Anyone got a part worn or milled out Browning slide and/or barrel to show what I mean?

The Enforcers are getting more mysterious every day..................................