The 22 Luger, almost but not quite.
 

When Mauser tooled up for production of the post war Parabellum, they wanted to offer as broad a selection of options as possible.

Early on they found that Swiss modifications to the design caused functional problems between calibers and barrel lenghts and it took a while to sort them out.

But the original serial numbering plan also shows a less obvious caliber, which I originally treated as a typo.

.22lr conversion kits are almost as old as the Luger and Mauser also experimented with the use of Erma conversion sets. Mauser was gambling on the bankruptcy of Erma in order to get the technology cheaply. Unfortunately for Mauser, the struggling Erma company actually outlived them.

The alternative .22 considered was called the .22 Lowenstein Luger, based on the .30 luger case. Rather obscure now, but it nearly made it as an official Mauser Parabellum round.

Shown are the original numbering plan for the Parabellum as well as a copy of the .22 Luger round details found in the Mauser archives.

The cartridge is "back", as the .22 TCM ? .223 case necked to .22.
The .22 9r is shortened enough to fit in a 9mm luger case.

Seems there are no "new" ideas- just recycled oldies but goodies. ;)

The .22 9r is on the left in the picture.

Actually, the 9R is on the right.
Here's my 9R, a CZ variant.

https://i.imgur.com/fmI4JxR.png
https://i.imgur.com/v6TuAW7.png

It's a fun little pistol.