Portable bunkers like this were intended to quickly supplement static defences with hard points and would most likely have been dugin so only a small part would be visible, and have mostly been around since at least the 19th century so considerably before WW2, this version is just another spin on the concept originally I believe started by Gruson in the 1880s...

http://www.lonesentry.com/articles/mobilepillbox/index.html

Colt .45 pistol as a fully automatic version. Some were carried by aircrew in the USAAF in WWII.

http://www.thefirearmblog.com/blog/2010/03/29/colt-m1911-machine-pistols/

Mike Bunkermeister Creek
Bunker Talk blog