'Tis, indeed!

Here's one on a 'Mousie', from the 1939 Stoeger Catalog:

It was a $20 'cost option' for Stoeger imported Mausers, only $12 to add one to a Mannlicher Schönauer:

The 'housing' looks a bit different on the Mauser than on the MS version, but otherwise they seem very similar. Does anyone here own one, or has anyone seen one of these on a Mauser?

It seems they are mostly found on the 'takedown' Mannlicher Schönauer models of M1903 through M1924. The one offered for sale clearly shows the 'lug' that the takedown MS action fits into at the breech end: