I have a pair of 67 4299s here, 3 step rods
It actually has nothing to do with the 3 step rod. Plenty of carter 3 step rod carburetors use the standard AFB 2 step rod piston. There are a handful of carbs using this style piston with added spring. 4131 is one besides these 440 afb from 67. I do not believe I have seen an AVS with 3 step rods use this piston. But there could be a model that does.