UTLX Tank Car ?s

I am working on frameless tank cars modeling the early 1950s with the 88100 series numbers.

My questions are;

1. How many of these tank cars were around then?
2. Did any of these cars not have the GRAMPS lettering during this time?

The info that came with my decals says the original number ranged from 55000 to 55999 and 57400 to 59999. In 1947 they were renumbered between 88152 and 88168. There seems to quite a bit of missing cars, what happened to them?
Then the info continues to say they were re numbered again in 1956 and the numbers ranged from 11034 to 11058 now there is more cars than in the late 1940s?
Any clarification would be greatly appreciated.

Thanks
-Kevin