I wish I could answer your questions. I bought the bike without the petcock. PO had clamped a hose to the drain hole which ran into a fuel filter, ran a hose from the filter to a T-junction and then into two seperate lines running to the carbs. Not pretty, but it worked. So,,,,,,, long answer to your question, I am not sure what type should go on it.
Any '75 550K owners out there wanna help? I am also wondering if any of the Dime City Cycle ones will fit.