There are several different groups of numbers on a 901 trans. The cast-in numbers are part numbers.
There are stamped numbers at the bottom of the trans, either on the center rib or on a flat boss just to the side of the rib. (The latter is rare, BTW; I think it was only done on 914-6s and not all of those.) So of those might have the transmission type number, which will be different on a 914-4 than a 914-6, and yet further different on a Sporto. (Those are very very rare.) I don't remember the designations off the top of my head, though.
There were two different ways the serial numbers were added. The change-over was in the 72 model year, I believe. Early ones had an actual serial number, while the later ones would only say what day they were built.
OK, according to
901 Series Porsche Transmissionthis page[/url]:
The 914-6 had "914/01 75ynnnn"; the "01" could be "05" for a Sporto, the space could contain "10" for a ZF limited-slip, the y would be 0 for 1970 model year, 1 for 1971 model year; not sure about 72 model year; and the nnnn is the series number.
The 70-72 914-4 had "914/01 HAnnnnnn"; the "01" could be "05" for Sporto (but I don't think that was ever offered on a 914-4), the blank space could have "10" for a ZF limited-slip, the A would also be a "B" for the limited-slip, and the "nnnnnn" is the series number. (I believe a bunch of these did not have the "901/01" stamp, though.)
The 73+ 914 had "HAddmmy", where the "A" could be "B" for a limited-slip, the "dd" was the day of the month, "mm" was the month, and "y" was the last digit of the year of manufacture.
--DD