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).
The specifications for a progressive cap, detailing pitches and coil/slot configurations.
| Identifier | ExFiles\Box 68\4\ scan0179 | |
| Date | 8th June 1928 guessed | |
| (13) (5) Progressive Cap. Pitches forward 7, coil 3 slots. backward 5; coin 2 slots. Step 1 to 4 Up 1 3 5 7 9 11 13 15 17 19 Down 8 10 12 14 16 18 20 2 4 6 | ||