The B18 and B20 have the oil pressure switch in exactly the same place,
right behind the exhaust. The manifold brace is in front of that,
and the other big threaded hole is used on some FI engines for an adaptor
for the thermal timer, but like Yogui says, is also handy for swinging the
engine. That one is further back. Go about 2" up and 4" forward from the
oil filter and you should find the switch. (Maybe it's broken off?)
--
George Downs Bartlesville, Heart of the USA!

Pearl;

Last pic shows the location of oil pressure sender.

The first two are generally unused (except as you noted, on later B20s)...they are threaded, so I've always suspected that the factory threaded these holes as one of the first operations, then bolted the otherwise raw engine castings to fixuring for dimensional consistency during the rest of machining operations...

Cheers