[MRCG] Humidity instrument...no internet presence

[Craig VonIlten]

So...I've taken up a new (related?) hobby...reloading.  So...blending some MRCG interests, I picked up an old Army temp/humidity meter.  I cannot track down any information on this animal.  It is in pristine condition...with spare parts and a microscopic schematic inside the base (where the batteries go).  Here is the only information I have:

The instrument is labeled "Indicator, Humidity Electrical Resistance" ID-421/U, serial number 141, Order Number 07131-PP-59.  Made by Monmouth Electric Company, Inc., U.S.  

The short instruction label inside the lid references tech manual 11-6685-204-12

The bottom right hand corner of the schematic appears to have: TM 6685-204-36-4.  The "36" could be a 35, 38 or a 39.  (The entire schematic had been shrunk down to a 3x4 inch piece of varnished paper).  This runs on 1.5V D cells for filament and two 67.5V B+ batteries (in series) for the B+, driving three 1U4 pentodes and an Amperex 1M3 "eye" tube (very interesting tube).

I can provide photos for those interested in helping me to sleuth this out...

73

Craig
N6CAV