I think I answered my own question. Back in the day, when these parts were the shiz, the clever engineers at Vicenza made two sets of chainrings, designed to work together. 53/39 was the standard road configuration, and 53/42/30 (or 32) was the triple. The 42 has ramps and pins to help get up from the little ring, and the 39 does not. So...oh, well. I rode a 42 for a long time before 39s were cool. I suppose I can deal with that. But the little chainring has no pins. I can change that out, right?