Over the past 57 years, the master cylinder might have been replaced more than once. It was common for people to simply put in whatever available part fit (usually a used part, if one was available - and, cheap used parts were readily available). In the late 60s, I worked on these trucks for a dealer. At that time, these trucks were considered to be junkers and it was common to do the cheapest repairs to keep these trucks running at the lowest cost.
There were two 1/2 ton master cylinders in 1954: the one used on manual-transmission trucks and the one used on Hydra-Matic equipped trucks. I think they both had the same bore but I am not certain of that.
If you checked the 1954 Shop Manual, you saw that the bore was 1 1/8" on 1/2 ton through 1 ton trucks.
1947 MC specs 1948-52 MC specs 
