There’s a tool in Stradivari’s collection of tools that’s designed to prick depth marks to be used in the graduation of the plates. It looks somewhat like the sketch above, which is a drawing of the device I made for use in my own shop.
It is used to punch holes to a depth that leaves unpunched the desired graduation thickness. Usually Stradivari removed all of the marks it made on the inside, but not always, as you can see on the inside of this unusually-well-preserved Stradivari top:
In softer wood or if the punch point had to go through too much wood, the anvil on the other side (outside) could leave dents in the surface. These can be steamed or scraped out, but once in a while they’re visible.
This cello back has them all over, and these aren’t the only ones I’ve seen on Strads:
Notice, also, that the arching on this cellos is so full at the edges that there are clamp “smiles” all around the c-bout just inside the purfling where the clamps used by some later repairman to glue the back on bit into the rising arch.
Here’s a close shot of a couple of the punch dimples: