I was wondering about that too.
Sometimes, if you click on the link, to the left of the button to increase the size of the photo in Google Images, it will lead you to a page that provides more info.
I honestly don't know who is using it that way, but suspect perhaps it is the Danes, since they apparently wore surplus British uniforms after WWII, and some of their squad leaders, and/or others (?) used SMGs.
They also had a lot of American weapons too, so that seems like a probable source.
Ah, looks like perhaps I'm wrong, given the Canadian pamphlet (lots of good photos of it in use here as well):http://regimentalrogue.tripod.com/blog/index.blog/2335550/35inch-rocket-launcher-m20-super-bazooka/
Here's a quote from one of the links at the bottom of the page, on the left:
"Super-Bazooka can be fired off the shoulder, thanks to the skeletonized shoulder stock attached below the rear part of the barrel, and an optional folding bipod can be fitted to fire weapon from prone, supported position. Some Super-Bazooka launchers were also provided with retractable support monopod, located in front of the shoulder stock".
Finally, here's the link to the Field Manual for the 3.5" Bazooka (it's a free PDF):http://www.koreanwaronline.com/history/Bazooka/1.pdf
Page 3 of the M20 Field Manual shows photos of the standing, sitting, and prone firing positions.
The latter is aligned to the weapon like your M72 LAW gunner is (legs about 70+ degrees away from the weapon's barrel axis), so perhaps he might be just modified a bit to make him a M20 3.5" Bazooka Gunner firing from the prone position.