my mate has a rb25de, the head has a completly different number cast on it. They are sure it's a silvertop rb20. I wondered if the redtop rb20det-r (gts-r engine) had different ports. the old L1(small) head is off the engine, and I'm going to have a look at it and get details on it soon. But the owner is 100% possitive that the orginal (L1 small) head and new (L2 big) head are both silver tops. Cause a few of us have asked that question.
Be interested to see if anyone has any unported silver top rb20 heads laying round to compare.
Was the R32 RB20 ever raced, cause anther theory was it was a homligation/cheater special. I know Subaru got EJ25 blocks cast to look like EJ20's for the factory sti rally cars (a local bought a ex crocker car, and couldn't get ej20 head gasket to fit, ony a ej25 would fit).