I'm looking at a Canadianicon National Defence publication titled:

First Line Maintenance Instructions
Rifle, Calibre .303. No. 4 All Marks, with Equipment.
Dated 1991.


The apparatus and tools section does not list this specific gauge as far as I can see, however it does show a "Rod, Plug Gauge, Mk2", no diameter shown,
as well as :
"Gauge, Plug, Plain Cylindrical, NOT GO, .310 in dia"
"Gauge, Plug, Plain Cylindrical, NOT GO, .308 in dia" and
"Gauge, Plug, Plain Cylindrical, NOT GO, .307 in dia"

Could my gauge be an early version of the "Rod, Plug Gauge, Mk2" ?