Firstly, is it an empty case (no propellant) or is it Cordite? Only Britain and Empire countries made .303 with Cordite (Japan used Nitro powder).
The Bullet design ( and case neck stab crimp) put it in the 1907 to 1940s time frame ( mid neck crimp, CuNi Bullet jacket.)
UN-ringed (crimped) primer: Common in British production up to middle of WW I;
Flat primer Introduced WW I ( original primer was rounded, and partially flattened by seating Punch)
And the Most common “Unheadstamped” Ball cases are found in WW I production ammo. Cases were Head-stamped separately from Heading, so cases could “escape” the HS stage, quite legitimately, and still be loaded.
In late WW II, RG also made unheadstamped L Mark V blanks. ( from packet Label ID).
IMO, Just a manufacturing Variation, probably by any one of the WW I British .303 makers…is there a case maker ID letter inside the case between the two Berdan flash holes? does the bullet base have any design or ID letter?