I have a 2001 cav z24. I removed the filler cap for the first time in 104,000 miles and noticed that it is a two piece cap. Either GM did it or the dealership did it, but on the mating surface of the cap is an alignment paint mark and I noticed that it has a hole inside the threaded neck. I am assuming that it is a breather cap. Is that correct? Well thanks for reading and for any info you can pass on.