If it is a base model GTR, then they are all the same IMO.
There have been a few refinements with the RB26's through out the different models + the engine producing slightly more torque, but that will be due to the tuning of the car.
Even with the slight tweaks here and there, it's not enough to say it's a totally different engine.
Visually the Cam / rocker covers are Red (R34 GTR) VS Black (R32 / R33 GTR)
The Letter "RB" and the numbers "26" are a different font style on the R34 GTR, but that's nothing to be concerned about.
But if you come across a N1 RB26, it is different to the base model engines.