Ok, I know that I have mentioned this before, and people have not believed me or wondered what I was talking about, so I finally go a picture. Here are the two different vintages of ETC Source 4 lamp caps. The early version (fig.1) has four pins on the fixture (circled in red) that mated with the hollow stand-offs on the cap (also in red). Fig. 1: On the newer versions of the fixtures (fig. 2) these pins do not exist, so the stand-offs on the lamp cap are solid. Why this change, i have been told that it was a change in the casting process, and the pins were there for the machinery to grab the fixture by (or something to that effect). Fig. 2: In any event, this is why a new lamp cap will not fit on an old fixture, the cap cannot seat correctly because the stand-offs are solid. Yet an old base will fit on a new fixture. However, I have been told by ETC that there is no reason not to grind off the pins on the older fixtures to allow for swapping bases.