My guess is that the forend latch was a push-button by the looks of the barrel attachment and the small hole in the off side.
It would have a pin with a wider diameter at the distal end and a smaller diameter near the button so as the button is pushed, the barrel is allowed to drop into place and then when the button is then released, the larger diameter portion of the pin intercepts the barrel attachment and secures the barrel. Then the lever at the front of the magazine box secures the action.
Very cool rifle!
Can we get a look at the barrel channel where the barrel attachment is?