12×12 mat —
6.00″ × 10.11″
ECMA B20.04 with FEFCO 0435's stacking tab. Each corner closes on a diagonal fold — a web — so the walls rise together with nothing to lock. The two end walls fold back down inside themselves, and a bump on each folded edge tucks behind a tongue slit into the base. Two more tabs stand above each end wall's top edge and come up through the base of the tray above, into the slit that tray's own lock uses. Ours, not a catalogue code.