Common cable types explained...
VIR: Vulcanised India Rubber. Single-core unsheathed cables used in conduit. Rubber insulation has an outer fabric layer impregnated with a preservative compound. Tinned copper conductors, stranded in all but the smallest size. Conduit was normally black-enamelled. Domestic installations often used 5/8" or 3/4" slip conduit that was not threaded at joints but clamped by the fittings, a.k.a slip conduit. Continuity required enamel coating to be scraped off and was considered unreliable with cheaper fittings.
TRS: Tough Rubber Sheathed. Construction generally similar to modern T+E, rubber insulated cores with or without a bare CPC, inside a rubber sheath. Usually black, occasionally white. Tinned copper conductors, stranded in all but the smallest size.
Lead sheathed twin: Rubber insulated cores with or without a bare CPC, inside a grey lead metallic sheath that serves as CPC unless a copper CPC is included. Tinned copper conductors, stranded in all but the smallest size. Original work will be jointed in metal junction boxes with continuity clamps that connect the lead sheaths together, rather than bakelite ones.
Imperial sized PVC and PE insulated twin / twin + earth: Construction generally similar to modern T+E but with imperial sized conductors, either tinned or plain copper, stranded in all but the smallest size. Some early plastic-insulated cables used polyethylene insulation which has a smooth, waxy appearance and may be semi-transparent.
Aluminium conductor: Construction generally similar to modern T+E. Conductor soft and brittle regardless of whether copper-clad. Cladding was to improve contact at terminations, plain Al makes poor terminations that tend to go high resistance unless special precautions are taken.
Imperial sizes:
Example: 7/.029 (note position of decimal point carefully)
7 strands of 0.029 inches diameter.
CSA (0.029/2 x 25.4)² x pi x 7 = 3.0mm²
Normal strand diameters: .029, .036, .044, .064, .083
Normal strand counts: 1, 3, 7, 19, 37
Common domestic applications:
Lighting circuits: 1/.044 or 3/.029
Power radials: 3/.036 or 7/.029
Ring finals: 7/.029
Cookers and submains: 7/.044
Tails: 7/.044 or 7/.064
There were other / older systems for specifying cable size, e.g. 3/18 meaning 3 strands of 18SWG. These were not normally used after WW2.