Snail magazine surprise
 

There is always something new to learn. I always thought it was made only for arty Lugers.

It was made for the LP08; the MP18 was partly designed around the mag to use the TM08.

It requires an adaptor for the MP18. It is just a simple sheet metal tube, kind of like an open ended dust cover.

Although Hugo Schmeisser, part of Theodor Bergmann's original design team, came up with a 20 round box magazine for the gun, the German arms commission insisted on the use of the Trommel magazine, since they were in the inventory and relatively plentiful.

I think the changeover to the 20 round box magazine occured during the Weimar era and guns so modified usually have the 1920 double stamp on them.
The sleeve was designed for preventing the Trommel magazine from going too far into the action of the MP18. Why they did not simply design the gun to take the magazine without the sleeve is a mystery.