Full Details

Name and Model Number	Key to the steel square
Country of Origin	USA, Lincoln, Nebraska
Date of Manufacture	Copywrite 1902
Front Scales	Rafter table scales
Back Scales	Table of Tangents
Scale Length	2 inch diameter – 6.28 in scale length
Type of Rule	Disc
Construction	Printed on plastic
Area of Use	Generic
Comment	Used for measuring and cutting rafters and using the steel square. Cost $1.50 in 1902
Rarity	RR
Donor	Collection of Clay Castleberry
Match Number	313
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In conjunction with our 'donors' we assign a rarity rating to each item, based on the system devised by Herman van Herwijnen

(1929 - 2004), renowned Dutch collector, author, and creator of Herman's Archive, a collection of 5,000 slide rule images on CD.

• RR = Rare - 1 of 25 collectors has one; it may take a year to find an example in the marketplace.

• RRR = Very Rare - only a few known to collectors; may be several years before it comes up for sale.

• RRRR = Extremely Rare - a museum piece; unlikely that an example will come up for sale.