An interesting 'naughty' Irish Trainer, Rifle No. 2 MKIV

[Flying10uk]

You could remove the screw, measure the diameter to see if it appears to be a "standard size" metric screw which I suspect it may be.

If it's not an imperial or gun thread size thread then I would say it's a relatively modern fitment.

In general engineering terms the use of imperial size screws in the UK was continued for a long time after WW2 in engineering.