.303
Yes, both those are correct.
What do you want to know? Portugal used .303 from WWI until the mid 1930's, but in latter years FNM produced amm on contract for all sorts of people including the British (7.62 x 51mm and 9mm).
Greenwood & Batley are one of the most important ammunition companies in the UK, as they made much of the cartridge making machinery for countries all over the world as well as the ammunition itself.
For example, the .303 Rubin rounds used in the Lee Metford 1888 Troop Trials were made by GB, and they produced tens of millions of rounds in both world wars, ranging from .303 to 15mm BESA.
They also used the headstamp code "G" (as well as "GB") on the early marks of .303. I have 1888 Rubin, Powder Ball Mark II, Cordite Ball Mark II, IV, V, VI, VII and VIIz by GB. AFAIK, they only produced ball and dummy in .303.
Regards
TonyE