[GreenKeys] Telephone equipment factory question

[Alf Fisher]

Hi Greenkeyers,

Has anyone ever heard of sound levels being measured in mm?
Could this be millimetres or some other quantity?

In America in the 1920's, a noisy factory was making telephone relays for 
Western Union.
They had 470 coil winding machines running and the sound level was recorded 
as 0.85mm.

Does anyone know how this would equate to the dB sound levels we use today?

Regards,
Alf Fisher
G3WSD
Bedfordshire
UK