Probably been asked too much, but I just pulled out my old jets to return to the stock size. I pulled out holley jets that say; 67H. It also looks like they are black. Does this have any significance over stock brass jets with only a number stamped in?
Some of the letters meant close hole ID tolerance between the two jets (+/- .5 to 1% or so as compared to 2-5% normal) and some letters were the manufactures ID when all the outsourcing took place. The black comes from the machine coolant (oxidation of the brass) that was not properly washed off after the machining operation took place.