You would be better getting it off the engine itself incase the engine has been changed.
My engine number is RB25 XXX XXXA (RB25, then 6 numbers, then a letter)
Edit: just went and had a look, my RB20 is also the same (RB20, then 6 numbers, then a letter)
Edit Edit: Its on the passenger side of the motor right down the bottom near the gearbox on a machined surface. You might need to get some degreaser onto it.