I bought an 18R carby & manifold for my RT104 Corona today, but it came off an RA28 Celica. When I bolted it up, I noticed that it didn't cover this hole - which from memory didn't exist on the RA28 head. I assume it is something to do with pollution gear, but if anyone knows where it goes, I'd be interested to know.
I also want to know what models of carby 18R heads came WITHOUT this hole as I now need to get one of these