The cross pin I see on mine is on the RH side, facing front-to-back, and can be seen in this pic:
That's the only cross-pin that I see. Looking more closely, I can almost bet that is how it comes apart. It seems that cross pin is driven out, then the vertical hinge pin - the main one - is driven downward from the top. The latch mechanism appears to be designed so that it will move to the "unlatched" position (button engaged) when it is hammered out. That's my guess. In that schematic, the parts that will be hammered out are shown down behind the pistol grip. Appears to be a spring and that long stepped shaft. The pin behind it is what keeps the stock tubes installed on the rear boss.
The little front-to-back cross pin that starts this off is shown just to the left of that guy's blue arrow.