Paul,The numbers you refer to do not correspond with the years you are describing. For the mid 50's there was no number code, it would start with something like EBU or ECZ. in the later 50's they came out with the numbering system to correspond with the years.
IE. B9AE translates to B=1950's, 9=year 1959, the AE I believe stands for Auto Engine.
In the 60's the everything made starts with the letter C.
IE. C1TE = 1960's era, year 1961, TE I believe stands for Truck Engine.
so the numbers you are refering to from Eikman's book do not appear to correspond with 55 parts.
The "U" you refer to means it came with a 272.