H&K is not a specialty of mine, but I've handled an MP5 once (New favorite gun, oh my god you can write poetry on the wall with that thing on full auto.)
If I recall correctly, the MP5/10 (10mm auto) and MP5/40 (.40 S&W) use straight stick mags. Usually polymer. While the MP5 standard uses slightly curved metal mags. From this picture I really find it hard to tell which magazine that is. It could be polymer, and it looks a little chunkier than I recall the 9mm mag being. So it may be one of the others. But.... MP5, close enough I'd say, and I'm usually a stickler for these things.
I've got a friend who actually works in the porn industry (Not as exciting as you'd think.) and I was over at his house one day when he threw a magazine on the table that was folded back to a page about 3/4 of the way through. Had an obvious paintball replica of a .50 barrett, but it was very very detailed and if it hadn't been for the translucent mag and orange barrel tip, it'd be perfect. I said as much to him, to which he replied. "You're supposed to be looking at the completely naked woman holding it."
Sure enough, I hadn't even noticed her. *shrugs* What can I say, I have priorities.
Enough rambling, must add something semi smexy.
http://youtu.be/wZp6NXl-clc