Gauge...
 

This is a firing pin protrusion gauge Pete...

1.1mm being the minimum allowable limit

...and 1.35mm being the longest allowable.

Thanks, John...

Any idea what the small u-notch at the top of the gauge is used for...?

I believe that this is a "shape" check for the very end of the firing pin, to ensure that it has not been deformed by firing or dry firing or some other misuse... Any sharpness in this area could lead to pierced primers... not a safe condition in any gun.

Hello Pete,
At page 60 of my book you can find some other examples of this firing pin gauge with the Waffenamt stamp but without the Mauser Banner.
The explanations about the use are available too.
Note that in some gauge variations the u-notch at the top is labeled with 0.75.
Ciao
Mauro
:bigbye: