| Name and Model Number | Keuffel & Esser Mystery 20 Inch |
| Country of Origin | USA |
| Date of Manufacture | 19343 |
| Front Scales | 20in / DF [ CF CIF C ] D | 50cm |
| Back Scales | Slide: [ Unknown Scale in Degrees ] ; NBS Circular No. 47 Conversion Table |
| Scale Length | 20" (50 cm) |
| Type of Rule | Slide Rule |
| Construction | Celluloid on mahogany |
| Area of Use | Generic |
| Comment | Mystery rule with no model number. Scales on front similar to older 4088-3;
scale on reverse of slide is calibrated in degrees and is folded at 31º 34'. Function of the reverse scale is unknown as is its reference point. Serial No. 460528. |
| Rarity | RRR |
| Donor | Collection of Dr. Paul J. Tarantolo Jr. |
| Match Number | 27 |
| Thumbnail (Click Picture for Details) |  |
| 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. |