We didn't worry toooo much about the aesthetics of things! Barring out was the norm as you see from Bren barrels etc etc. At Aldershot and Warminster we had several jigs specially made for things that needed numbering, such as No2/L9 slides and barrels, No4 magazines and bolts, SLR TMH's, BB's and carriers....., you know the sort of thing. Put the part in, tighten up with a couple of positioning bolts and they'd align it to a pre positioned place It was a simple matter to take out the engraving cutter and put in a small milling cutter. Mill out the old number and then engrave the number into said slot. Did plenty of No2 pistol slides like that too as we weren't allowed to stamp the number