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).
Four indicator diagrams at temperatures ranging from 5°C to 20°C.
| Identifier | ExFiles\Box 152\1\ scan0114 | |
| Date | 20th February 1933 guessed | |
| 5°C 457 20°C 460 10°C 458 15°C 459 | ||