The casting number 591615 was used from 1948 through 1954. However, they made internal changes in 1953 and 1954. The 48-52 use a 2 lug synchronizer and the 53-54 use a 4 lug synchronizer. The best way to see what you have is to pull the input shaft. If the synchro has 2 lugs, it is 48-52 and has the input and second gear to match it. If it has 4 lugs, it is a 53-54 and the matching gears.
Here is a parts illustration:

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0707.htm

Casting numbers can be identified with the Hollander. This is the book salvage yards used to identify parts.