Just throwing it on the pile. For the last few years, 1675 has been using "locking pins" similar to the "squared retainer" style
here, and 2 sets of bumpers. The pins go through 2 angle brackets from the bumper on the top and bottom of the frame and the frame itself. Sometimes it is a tight fit putting them on with components inside the frame but you can always bring the lock in at an angle as opposed to perpendicular.