hi oldrunner. Yes, the automatic is the big 71-series and the 4-speed is the little 56-series. The gears are definitely different. They were used by Nissans small and big into the late 1980s
Not sure what the bullet is. Nissan uses these terms:
Sleeve & Pinion
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Notice there are two O-rings
I wonder if oil leaking out is due to a worn speedometer cable? The cable runs transmission fluid inside it which is not supposed to leak out. Or maybe the seal is missing. Oreilly stopped carrying the 1200 cable about 2 years ago (it was only $15), but a new cable is $7 at
Rockauto. Use the B210 cable which fits 1200
New Sleeves and/or pinions are available, search for other Datsun, for example 280Z:
Datsun automatic