I think I know what the go is....
From right to left
1) RB20DET engine, they can only be used on a RB20 series engines, the top hole is not slotted and is slightly offset so it doesnt seem to fit other RB's.
Part# 21010-70TY5
2) R34 RB25DET NEO engine, this is also one of those pumps that fits the RB25DET, again, fitment on other RB doesnt seem feasable as the hole is not slotted and is slightly offset towards the left. The casting design seems slightly different as well, it seems to look slightly shallower then the other models.
Part# 21010-AA527
3) Standard RB26DETT and R33 RB25DET, this one is able to fit on any RB series engines ranging from RB20DE, RB25DE, RB20DET, RB25DET, and RB26DETT of all series. This one has the hole slotted to fit multitudes of RB engines, this is ok to use on any RB engine.
Part# 21010-21U26
4) RB26DETT N1 specification engine, this one fits the same as the one above, it fits all RB engines of all series, but what makes this pump special is the anti-cavitational 6 blade impeller instead of the others that have a 8 blade non backing plate designs. This pump originates from the Skyline R34 N1 models, this is a ideal upgrade for those guys who like to wind up the RB since this limits the coolant caviation to a minimum.
Part# 21010-24U27
I think the previous person who changed mine used 3), the standard R33 one or a VL one - which both have an elongated or slotted hole
The guy who provided the above info said
For those of you that install the RB pumps with the slotted holes 3) and 4), you must be very careful applying the silicone sealant around the slotted hole. If not enough is applied it will start to leak around that area as the surface area for sealing is very small. Pay particular attention to that slotted area when installing!
This is because on some cars, the bolt that goes through that elongatedor slotted hole doesnt actually fit in. Hence, if you dont carefully put enough sealant there, or its not done right, it will leak. In my car, looks like it is leaking there.
More info here http://zilvia.net/f/chat/229276-skyline-rb...o-id-101-a.html