.303 Marked Gauge Tool... identity?

[CINDERS]

#8 MHB if you look closely at pic #3 you will just be able to see there is a counter sunk hole in the end of the spindle that would allow it to go over the anvil.
Or further more perhaps it is for measuring the top of the anvil as opposed to the base of the cartridge as if it was to deep would allow the primer to sit a bit deeper which hopefully would still allow ignition but if the anvil was to high then the primer may not sit below the base of the cartridge or worse still proud causing all sorts of issues not only in the field but in production.
That's my take on it.