Removed cap and proceeded to break rotor, I though it just pulled off like on autos. Most parts are hard to get. I found a complete distributor on ebay which (claimed) came of a working 1966 merc 500. It looks identical to the one I have. I know the part number for mine is 393-2799A7 or A20. How can I find out the part number for the 66 merc 500 to see if it is the same?