THere were no official tools that the soldier/sniper could use or was permitted to use on the Mk3/L1A1. The Armourers that were Instrument Qualified used the tools that were issued and such a tool wasn't issued......... The tool that WAS issued was a Spanner, pegged, adjustable, V6-WZ 0058 which sort of did the job - if you were desperate! Most, like me, made their own tools according to their needs. I've still got most of mine that have been used for some 40 years now. But I never saw a Mk3/L1A1 tool. Indeed, an Instrument technician wouldn't need to hold the lead screw while he locked the locking nut down because.............. anyway. Once he'd set it up, they went onto the test range where the instrument techs and Armourers would have a play and then the sniper made the final adjustment using the nose of a round.
In short, nope......, I never saw or had need to use a Mk3/L1A1 adjusting tool. But I could be wrong