Oh I should have verified that...
"Prior to the introduction of Mk. II parts there was no need for model identification on the pistol, but with the introduction of the variation it was ordered that the applicable number and mark be engraved on the left side of the slide immediatley above the maker's legend. Pistols on issue were not to be withdrawn specifically for this purpose, but if, and when, pistols were in workshops for repair or servicing, the numbering was to be carried out. Stocks in depots were to be engraved at the time of issue. This means that if the engraving is "Parkerised" it was put on at the factory before the pistol was finished; if it is bright, it was done at an army service depot in accordance with regulations.
-blake stevens booklet