It seems that you're right, and if so it was proofed for 6.5X54 rather than 6.5X55.

Being unfamiliar with the 1954 - 1989 British proof system, I had to compare images of other firearms Brit proofed during that period.

My M1910 bears British proofs from the 1920s along with "9.5 M/M EX" and "Cordite 43 - 270 MAX ", which defines the 9.5mm projectile, a maximum load of 43 grains Cordite, and a 270 grain projectile - all quite useful information. I have seen many images of others marked in the same fashion.

Apparently from 1954 through 1989 case length and 'long tons' per square inch of pressure as determined by a lead crush method was more important to British proof houses than bullet weight and powder charge.

Actually 54mm = 2.12598 inch, but take note of the 2.110" case length of 6.5X54 MS on the (quite British) Eley drawing below.