Full Details

Name and Model Number Harrow Mark Reducer by Aston and Mander
Country of Origin England
Date of Manufacture c 1915
Front Scales C, D
Back Scales None
Scale Length 38"
Type of Rule Slide Rule
Construction Boxwood
Area of Use Other
Comment Proportionality calculator for grading tests.
Rarity RR-RRR
Donor Collection of Peter Fox
Match Number 60

RR = Rare -- 1 of 25 collectors has one; it may take a year to find an example in the marketplace.   
RRR = Very rare -- only very few known to collectors; may take several years before one comes up for sale.   
RRRR = Extremely rare -- a museum piece; unlikely that an example will ever come up for sale.