The TU stands for technical update on BMW engines, the OL indicates it shipped in a TDS package BMW (i.e is configured for intercooling)
Internals are the same on the m51d25 as far as I know between all the platforms it shipped in. Only difference between the early and TU BMW engines was the ECU, early had DDE 2.1 (ecu has a sticker on saying DIESEL) and the later models had DDE 2.2 (ECU is stamped DIESEL).
Assume they all ran variations of the same VP20 pump as well? Can't be hard to get one working.