| Name and Model Number | Billeter National Rechenwalze System 20-m |
| Country of Origin | Switzerland |
| Date of Manufacture | c. 1920 |
| Front Scales | Cylindrical C & D scales in 80 sections |
| Back Scales | None |
| Scale Length | 20-m |
| Type of Rule | Cylinder |
| Construction | Varnished paper scales on metal cylinder (20.5-cm dia. x 57-cm long) & on sliding cylindrical cage. Metal support stand. |
| Area of Use | Financial |
| Comment | Formerly Julius Billeter Rechenwalze. This cylindrical slide rule has a metal support stand that could be wall mounted. Commonly used by European banks for currency trading. This is a previously unknown size. The largest previously known National cylindrical slide rule had a 16-m scale length. |
| Rarity | RRR |
| Donor | Collection of Ed Chamberlain |
| Match Number | 154 |
| 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. |