The number you cite is a casting number. It is probably a 591618 which was the transmission case used from 48-54. However, 53 and 54 have quite a few different gears than the 48-52. The later synchronizer has 4 logs while the older one has just two. Two lugs can be ground off and used in the older transmission. However other gears will not mix and match. I haven't built any transmissions in a long time, but if I remember correctly counting the lugs on the synchronizer is the quick way of identifying the transmission. Unless you ran your transmission out of oil, it may not be as bad as you think. Worn shift forks can cause it to jump out of third. A worn out pilot bushing will also do the same thing.
