I enlarged the picture of the grenade launcher pictured at the beginning of this topic then made a tracing to show a little better detail.
The bottom of the rack is attached to the side rails with mortise and tenon joints and are secured by screws/rivets. The joints where the curved and vertical parts meet are probably made the same way.
The back rail and side rails are joined by a half dovetail lock joint. I assume the front rail, which is hidden by the metal strap, is joined the same.
The bottom of the metal strap on the side of the vertical rack part terminates just below the last rivet. The rivets on this part appear to be button head rivets. The other rivets appear to be wagon box rivets which have a large diameter, slightly rounded head. This type rivet was usually used when riveting metal to wood.
The trunnions appear to be set half way into the side rails and held by strap metal cap squares.
Not sure why the top three holes in the rack are there, since the barrel would block these holes when the barrel is raised to it's highest point.
The barrel appears to be resting on a blind dowel or something and there is also one in the top hole. Might be there to prevent people from moving the barrel when on display.
All in all this is a very well designed and fabricated piece. It shows wear and possibly some repairs.