You're correct, we've had this discussion before; there are examples of "new" pistons with the new P/N that are the old dimension but my measurement was taken from an original 623 piston.

In the factory list of running changes for the '74 model year they list a new "N-D (Neutral-Drive) package" that includes the new Belleville and a new piston with "an increase in the over-all height".