| 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 |
| Thumbnail |  |
| 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. |