This is an early Borg-Warner 412 T-10 four speed transmission. The build date is F 53 2 which is June (F)- 5th(5), 1963(3) and it was made on 2nd shift(2). The tag under the bolt holding the tailshaft reads 3D3-B-1. It has a 9 bolt Side Cover and 7/8 Counter Shaft. The 1957-1965 models use a Thrust Bearing between 2nd and 3rd gears, the '66-'69 don't use a Thrust Bearing. The last time I opened it up was in 1975 to replace the synchronizers and 2nd gear. I'm sure the shift forks needs replaced this go around along with other parts. To make a long story short, the T-10 drain plug is the only part that didn't need replaced. The clutch and 3 finger pressure plate has been a good one, it lasted for 20 years and still had some life. After inspecting the parts and doing the math I decided to replace the transmission. I found a Toploader on the Craigslist. The Toploader was not a direct swap but it was easy with the correct parts and a little machine work on the bearing retainer.
Old magazine on how to disassemble and assemble a Borg-Warner T-10