the top of the plate 5 and a similar series of latches 23 at the bottom of the plate 5 the latches 22 and 23 each comprise a latch member 24 pivotally mounted on a bar 25 similar to the bar 21 and extending between the vertical reinforcing ribs 6 at the front and rear edges of the stationary side wall 1 for pivotal movement between its latched and unlatched position as shown respectively in the full line and dotted line positions in fig1 the free end of each latch member 24 is formed with a catch 26 that is adapted to overhang the edge of the bar 21 and thus lock the movable side wall 2 against movement away from the stationary side wall 1 the latch members 24 of the latches 22 are adapted to be raised and lowered into and out of their latching position by a solenoid 27 depending from the frame and having its armature pivotally connected to a bar 28 underlying and connected to the latch members 24 the latch members 24 of the latches 23 are similarly raised and lowered by a solenoid 29 mounted on one of the channel members 9 and having its armature pivotally.