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Looking at various grenade launchers, this showed up. Original serial number, Irish ER serial and later G serial number (designated grenade launching). Brass disc indicates original issue was to a Royal Engineers unit. This would seem to tie the G series markings to Irish serial numbered rifles.
Breakup,
In my database I have 37 of these rifles, of which all but 8 are triple serial numbered. The other 8, below serial number G538 are all dual serial, that is they were set up as G rifles, not converted after being reworked as ER prefix rifles.
Yours is the highest serial number to show up to date. The next highest number is G2107
Yours, with the unit label is rather unique, no other rifle has a British unit ID.