I bought a used '79 (small cap) distributor off of ebay for a spare and because one of the screws is broken off in my existing one and it is definitely an electronic ignition distributor, but it has 2 vacuum fittings on the vacuum advance diaphragm. What is the other vacuum fitting for and what do I hook it to? Is there any advantage in this distributor over the one I have with only 1 vacuum connection? I hope I didn't buy something that I can't use. Thanks for any help you can give me!