What’s Loose? (need exploded parts diagram)

I have a small amount of play between the “upper case” (aluminum part that the picatinny rail screws onto) and the breech unit. I can’t figure out what fastener is loose that should hold the rear of the upper case down tight on the main body of the gun. Side to side and up/down motion is possible, and shouldn’t do accuracy or pellet seating any good as it is presently.

Regular Matador Long single shot in .22 cal.