Look at this. The pictures from Ron Scott.

You can see the push button sticking up through the rear of the front lid piece.

Here is the under side

Side view, upside down. You see the end of the pivot pin that the latch bar pivots on, a spring under it holds it out.

Now right side up. The pushbutton is pinned to the latch bar, pushing it in raises the latch bar up back at the butt plate end so lid can slide out. When slid in the latch bar compresses the spring and if made correctly just as lid reaches full movement forward the latch bar would clear butt plate and snap down to hold it in. Actually, it looks like the latch face might be a bit in from the butt plate.
Makes perfect sense, time will tell if I can do it justice, a project for next winter.
(It was in the 60's yesterday, 20's today and snowing, which does not qualify as next winter.)