NCbug,
There are basically 3 different types of 915 tranny.
The early ones have a mechanical speedo output and suffer from the input shaft seal (which is also the support for the clutch release bearing) pulling out and then you get oil everywhere...
It has 94mm PCD output flanges.
1972-1978 ish...
The next one has an electronic speedo output, looks similar to the early one but has the problem fixed with the input shaft seal, comes with 944/T2 output size flanges and comes in the 911SC.
1978-1983
The last one came in the 3.2 911 and has an oil cooler so is pretty easy to spot. It is the best of the lot as Porsche had ironed out all the probs. Output shaft size on this I can't remember as they're pretty rare.
1983-1987
The 2 earlier 915 trannys suffered from 5th gear seizing onto the shaft after very high speed long runs hence the oil cooler which dumps oil straight back in over 5th gear.
After 1987 the G50 came out and things were never the same again
Oh, the Turbo never used a 915 tranny, they had a 4 speed 930 capable of handling 800bhp!
MG