GB didn't put date stamps on the early stems. I think there were some serial numbers on later (>60's) stems though.

The classic spear-point stems had variously "Forged Hiduminium", "Solid Forged", "British Made" or "Made in England"

If it says Hiduminium then you should be OK for the 1950's, not sure whether BM predated MiE though. I checked the Brown Brothers' Catalogues: there is no mention of GB stems in the 1938 and 1939 books, but the spearpoint stem features in 1952, 1954 and 1959.

The 531 and Kromo stems were also available from at least 1952 - 1959. There were also 'girder style' stems from the same period, or from the late 1940's.

I think all other designs (including that posted by clubman) were at least 1960's. GB closed shop in the 1990's but not before producing the GB Biba stem, which is one of the best looking.

Some resources:
https://www.lfgss.com/conversations/175723/

Brown Brothers' Catalogue:
oldroads.com - Vintage Bicycle Reference
oldroads.com - Vintage Bicycle Reference

GB Components, main