Repaired shell

I spotted these two on the stems of dog roses earlier and have just returned with the camera, when zooming in it was possible to see the damage to the shell. Checking on line I came across the following on a aquarium site:
the snail uses glands located in its mantle and repair the damage by excreting ‘shell substances’ necessary to repair the old shell around the broken place. 
Within a few weeks (usually about 1 or 2 weeks) these cells become fused with calcium and harden the outer coat of the shell.

Those large thorns haven’t prevented the pair from climbing to the top.