591615 is the casting number for 1948 thru 1954 GM truck transmissions. It could either contain the earlier 2 lug synchronizer or the later 4 lug synchronizer. Chances are good that it is the earlier style, but the only way to know for sure is pull out the input shaft and look at the synchronizer. The shift forks were notorious for wearing and so was the synchronizer. Shift forks are relatively easy to find. Synchros are hard to find and expensive when you do find them.