Thinking out aloud Gary, if they were used in a serious front line role anywhere, I can't help thinking that there would have been an additional small-arms notice to the effect that '....... quartermasters and Armourers should be aware that quantities of rifles designated No1 Mk5 have been issued to augment existing stocks....... etc etc.....' You know the sort of things. Additionally, Armourers would have a parts list of different parts too. Otherwise they'd be out on a limb. I have never seen a note to that effect or even a parts list.
There was a note to this effect about non standard No4 rifles, identifiable by the now obsolescent cut-off (this'll be the trials rifles) and similar rifles identified as the Mk6 rifle (this'll be No1 Mk6's). But no mention of Mk5's
These little flimsy paper notices for the information of Armourers came out periodically as a sort of early amendment to the big instructions to Armourers. But being flimsy paper, very few seem to have lasted. I have one dealing with the very early repairs to Sten guns