One side of plate has 2 lower lateral stops and 2 upper symmetrical projections, the opposite side has a central lower stop and 2 upper symmetrical projections. Pref. the spacings between the stops and between the projections of the one side are such that when several frames are stacked, the stop of each upper plate fits between the projections of the plate below, and the other stops bridge the other upper projections of the lower plate.
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