Ok, it could be just me, but I have been noticing a bunch of very minute, but majorly inconvenient differences between lamp bases and Lamp caps on source fours. First off, apparently there are two designs of the rear of the fixture where the lamp cap meets the fixture. One has 4 pins that protrude from the fixture, and the lamp caps have matching holes allowing them to fit. This mean that a lamp camp without these holes doesn't fit these instruments, but the lamp caps from these instruments fit all others. The other thing, is, have they changes the actual porcelain lamp base? Because I went down to my supplier to pick up some replacement porcelain bases, and they only fit in the 750w caps. It is driving me nuts. Does anyone know if there really is a difference or what the two different part numbers are (ETC only lists one on their site)?