The cross bolt fully pierces the rib extension when closed. I have checked this visually several times. The inards were very filthy with hard grease and ferrous residue when I got it (maybe the guy got it wet and never took it apart?) and I have given it a good spritzing with gun action cleaner, worked the action, flushed again, and sprayed with Rem Oil. There may still be gunk in there, but as I am unsure how all the little bits fit together under tension, I am hesitant to disassemble too far.
But I have four other cross bolt guns to compare with and they all seem to react differently.
Husqvarna Model 51
Backaction model. Crossbolt returns to almost full closed position when gun fully opened. Upon closing gun, bolt retracts and rib extension clears with room to spare.
Husqvarna Model 103
This one is the closest to the 52 in quality and sidelocks. When gun is fully opened crossbolt returns to about halfway across mortise. When you close the gun, the bolt retracts and extension clears with no problem.
JABC No. 1
Actually a really nice and mint condition little gun. When opened the bolt returns to almost all the way closed. Upon closing the gun, the bolt retracts, but extension comes into contact with it a bit near the end and you have to snap it shut.
JABC No.2
Well-worn, but tight gun I shoot BP in for cowboy action shooting. When opened the crossbolt stays in full open mode. When gun is closed it drops back through the hole in the extension with no resistance.