Here is what the cab mounts look like:
First picture mid 49-1955 1st series:
http://community.webshots.com/photo/2379203940102454676qNTFMD
That is where the eight rubber donuts go.
Second/Third Picture is the center mount shackle 1947-Mid 1949
http://good-times.webshots.com/photo/2770506040102454676cTSfPv
http://good-times.webshots.com/photo/2433221960102454676NebfQM
Easiest way to tell is gas tank in cab uses the newer style.