From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Cost breakdown for a main switch, push button switch, and starter motor.
| Identifier | ExFiles\Box 50\1\ Scan049 | |
| Date | 8th December 1921 guessed | |
| X.4289 (14) Main Switch (X.3034). Present works cost of production £5.13.10 Should not be materially affected by change to series connection. Push Button Switch (X.2641). Present works cost of production £1.16.0 Starter Motor (X.3082). Present works cost of production £44.14.2 | ||