This is the basic plug-in logic module which surely has to be the ancestor of the PCB. Each type 12AU7 double triode valve stores one bit. A variety of turrets built to this similar physical design served all the electronic processes required, and the 550 machine contained 600 turrets. The turrets plugged into backplanes mounted on the six hinged 'gates' with 100 per gate - see the lower picture. Note the crudity of these hand wired units. In particular the size of the two striped 'miniature' metal oxide disc 'computer diodes' seems so huge compared with the equivalents of 40 years later. (This turret is in a distressed state : all the wires leading to the plug have been disconnected.)
