Full Details

Name and Model Number Proell Pocket Calculator
Country of Origin Germany
Date of Manufacture ~1901
Front Scales Base scale 15cm x 10cm Scale 1-10 split into 10 rows. sliding scale 15cm x 10cm as base but inverted.
Back Scales Basic instructions
Scale Length Length of calculating scale 120cm
Type of Rule Other
Construction Card base scale & celluloid sliding scale
Area of Use Generic
Comment A grid iron rule.
Manufactured in Germany, John J Griffin was the Great Britain agent,
GB Patent No. 2516 1901. Complete with slip case and instruction booklet.
Rarity RRR
Donor Collection of Peter Fox
Match Number 51

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.