It does sound like you have gearbox problems and they are not the easiest box to work on. They have to be split in three places, the clutch housing has to come off the gearbox and the gearbox has to come off the rear axle. I had to do mine last year because a tiny roller bearing failed.
You really do need the workshop manual to attempt the job. Might be a good idea to drop the oil out and see if any bits come out. I stripped mine right out and did not find anything until I took out the final shaft. Mine would go into all gears OK but knocked like mad in 5th and 6th.
It is just a three or four digit code on all cast parts, the easiest one to see is with a series of numbers on the side of the gearbox. The numbers are meaningless except for the code itself.
The casting code is from a 1964 tractor which has a number for the year.