the locked second and third frames b and c towards the first frame a is then effective for causing the outer camming surface 53 of the lock finger 50 to slidably move the latch plate 57 outward against the force of the spring 59 until it is completely withdrawn from the circumferential groove 60 of the third frame c fig3 and 11 then the third frame c retracts inside the second frame b thus the apparatus of the invention includes a longitudinally extending finger 50 carried by the first frame a a latch member 57 carried by the second frame b and transversely slidable either inwardly or outwardly within guide member 55 and across the end of the second frame b the latch member having a central opening 58 and the finger 50 having an inwardly curved end 52 which normally extends through that opening for controlling the position of the latch member 57 a spring means 59 urging the latch member 57 inwardly the latch member 57 normally occupying an outer locked position in which it engages the surface of the third frame c with the curved finger end 52 preventing longitudinal movement of the latch member 57 and thereby locking the first and second frames a and b together a recess or groove 60 in the surface of the third frame c which is engageable by the latch member 57 in an inner locked position so that the latch member 57 then locks the second and third frames b and c against relative longitudinal movement and the outer sloped surface 53 of the lock finger 50 acting to cam the latch member 57 outwardly whenever the.