I just had a look at my -11/20 (the two are basically the same machine; the /15 was intended for the OEM market, the /20 the end-user), and it has an intermediate between this, and the final incandescent bulb form (as on the -11/45), where there were bulbs with plastic bases plugged into sockets. The -11/20 has the same bulbs, but apparently soldered directly into the panel; I looked at the prints (it's in the 'KY11-A Programmer's Console' stuff - I see Manx says the prints aren't online, I'll have to scan my set), and there are bulbs in the parts list, but no sockets. Per the prints, the later ones were Benelex, an early form of MDF (although some -11's later used thick sheet aluminium). Noel