Sorry... I’m sure I used the wrong terminology now. I “think” I was talking about the recoil buffer (spring) that usually runs from the back of the bolt carrier back into the stock. I know that on the one I have that when I knock the two keeper pins out of the rear buttstock and pull it out, there’s a LONG spring that comes out with it. I’m guessing the one on that pistol model in your photo is shorter and stouter? :-)
The operating spring guide rod (the long spring and rod in the top tube) is about 3" shorter than on other H&K rifles, because the barrel is much shorter, so the rod has to be much shorter so it can be affixed to the reset of the receiver. Other than that, everything is identical with the regular rifle.
Except a weapon built as a handgun, then the recoil rod assembly, including the rear cap, must be designed so that attaching a rifle-type stock is physically impossible. A vertical foregrip could be added by anyone, but that makes you guilty a the BATFE-defined crime of "presumptive construction". BATFE said that a person possessing certain weapons, and parts needed to convert it to something that they have declared unlawful was already de facto a crime, even if you never bring the parts within a thousand miles of the weapon.
People should also be careful about possession of certain parts unique to the select-fire M16. At one time, the only forbidden piece was the full-auto bolt carrier, although a couple of smaller items needed a couple of surfaces ground down in order to guarantee they functioned identically to the semi-only parts.
Last I heard, BATFE declared possession of even a single unmodified full-auto component such as the main sear was considered the legal equivalent of possession of an entire select-fire weapon, even if that's the only piece of the rifle you've ever seen.
Stuff like that is mainly used as a crowbar to get a suspect to "cooperate" by becoming a snitch, or infiltrating some nasty, violet group, for other federal alphabet agencies, instead. The biggest "product" of the BATFE is providing warm bodies eager to cooperate with other agencies, rather than putting them in jail because of what they possess.