Except where noted photographs were taken on 20th May 2009 at the Bristol Robotics Lab. and show spare parts from the original 1951 FoB batch of Tortoises as well as spare replica chassis. Some of the original 1951 valves were used for the 1995 replicas.
 original valves |  replica chassis |  original gears |
Original FROG RevMaster motor modified with bearings for the rotor shaft but minus its brushgear.
For comparison, two Frog RevMaster motors I have together with an original leaflet.
Both motors have worn bearings and commutators and one brush-holder has deformed with heat.
90CG Photocell (P.E.C.) and DL93 Pentode
Relays
From GW's letter 27 Sep. 1954 to Mr R. A. McMurtie, Aukland, which also explains why rear wheel drive will not work
"The tubes are Mullard DL93 and the photocell should be sensitive to artificial light, the most suitable type being Mullard 90AV. The relay should have a resistance suitable for the miniature pentodes, mine are 10,000 ohms and operate with a current of about one milliamp. The contacts should of course be adjustable and be sufficiently robust to carry the starting current of the motors, which is usually of the order of 500 milliamps."
I find it interesting that GW uses the American term 'tubes' and not the British 'valves'. Later in 1960 he uses the same terminology to a student in Dublin.