The Birmingham Proof House date code (X-B / 2) 1950 rules says the Brit proof was done in 1972
X- 1972
B- Birmingham P/Hs
2- Individual Inspector's #

No way of telling the Month within 1972, They only changed the Letter to indicate the Year change every JAnuary

Proofed for 9x56 M/S cartridge at the time.
2.22 in the cartridge case length in inches.

Of course it could have been altered since then.