Donegan Optical Company Inc
Optivisor

3-1/2x, 4" Focal Length
Walthers Part # 240-410, p. 856 Walthers 2012 HO Scale Reference
A scale, $55.45, currently in stock at Walthers

 
The OptiVISOR(R) is a precision-made binocular magnifier that is worn on the head leaving both hands free. Can be instantly tilted downward when needed and upward when not in use. Can be worn over regular prescription or safety glasses. Comes with dial-adjustable, conforming headband, ABS high-impact comfort band, optical glass lenses mounted in an interchangeable frame. Six lens powers available, plus an attachable auxiliary OptiLOUPE(R) lens (all sold separately) for additional magnification.
 
For extremely fine work.