The model known as the "English take-down model" has a grip-cap that, when you press a small button at the back and to one side of it, will let the cap swivel 90 degrees and then drop down. This reveals a small cavity that is big enough for a spare front sight. If the sight is a little higher/lower than the one fitted, which was the case in my M1910, then you have some options with sight height for different loads. My one was wrapped in paper to stop it rattling, I suppose. It's something I've continued to do.

In those days, 1903-1945 of thereabouts, I'd think that anything could be had custom-made, so I'm sure this feature appeared on other models too on special order. But if you don't see that small button at the back of the grip-cap, then I wouldn't think there'd be anything underneath. Mind you, I've heard of people putting their old-style firearms permits and suchlike paperwork under the butt-plate of various other rifles, but never found one.

My German friend, who deals in classic firearms, thought that GK would stand for gerade Kolben, meaning straight stock. So the stock would not be Monte-Carlo style. I would have thought they were all GK in 1920.