Well... The very FIRST mechanical change to the hammerless Parker was only about 6 months after it was originally produced.
Originally the HAMMERLESS Parker did not have rebounding hammers. Instead the hammers were drawn back away from the primers by two tabs on the lop lever linkage cam. The tabs pressed back on a cut on the inside of both of the hammers. The hammers were drawn back by the opening of the lever and then the gun opened and cocked as normal.
Very quickly, this feature was done away with and hammers with the rebounding stirrups were used.
The patent that covers the hammer "lifting" feature on the hammerless guns was 368401. The design was replaced by rebounding patent No. 412340.
See here a photo of the "lifter" mechanism as I call it.
Old on left and newer on right.
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