It appears to be some type of go/no-go gage for the model 98. It's more than likely a firing pin protrusion gage. The groove on the top would be to check the shape of the firing pin tip. The notch on the right end corresponds to the correct minimum acceptable firing pin protrusion (.055" to .065") for reliable ignition per Kuhnhausen's Mauser Shop Manual.
1.4mm (.055") or greater is acceptable (Gut - good).
Less than 1.4mm and greater than 1.2mm is marginal.
1.2mm (.047") or less would be unacceptable (Aussch. or Ausschuβ - reject).
Schuβwaffen 98 translates to Firearms 98.
Is it rare? I don't think it's common. It's from the war period and would certainly interest model 98 collectors.