Casting 591907 is part #591908 1948-52 All (4 speed)
Casting 591907 becomes part #591954 1953-54 All (4 speed)
Part #591954 becomes casting 591953 1948-57 All (4 speed)

Pics of the new 1949 Chevrolet truck
four-speed synchro-mesh transmission.
http://chevy.oldcarmanualproject.com/booklets/49fssto02.htm

The link to view the contents of this booklet is incorrect.
The correct link is:
http://chevy.oldcarmanualproject.com/booklets/49fsstoindex.htm

-Tom

1950 Chevy 3100 w/ 1956 235