| 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 |
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| 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. |