All I have for C1666 is a stamped but not overprinted Instruction Manual. It does have a lube diagram dated 10/3/38 though. My scan is dated 2006. I seemed to be almost falling over Norton WD literature at that time. No longer, regrettably.
This C1666 manual was rather outdated already as the first issue of the drawing was in January 1937 while C1666 was built in 1938
The Vokes C.1a Protectomotor air cleaner shown in the lubrication diagram was deleted 2 contracts earlier than C1666.
Only contracts C8496 and C8753 had those air cleaners, both 1936 model machines.
Although it is clearly given in Chilwell admin I am still wondering about the C.1a definition. Is it really just a model code or is it an incorrect notation for C.I.A, being the Chief Inspector of Armaments??, C.I.A is on the lube drawing.
I have not been able to find the C.1a code in what is left of the Vokes archives.