is it on both case halves?
If you're just looking at that little ridge, that's to smooth the transition to the skirt. like theres still a ridge on the pa2's; it's just set back and serves not as a physical impediment to derestriction but is actually a flow improvement. I think.
another case difference on the 1's is the little hole for the reed block, theres a nub on the reeds too, and on the pa1's it's smaller so only the 1 petal reeds fit. so the reeds wouldve had to be modified to fit a PA1 case if it was ever a swap over. most people grind the nub off but you could drill the case hole bigger too i guess