Prismatically milled base, scraped faces, 1 horizontal and 1 vertical vial. Lengths 160 mm/200 mm/ 250 mm/300 mm.
| Design | 160mm | 200mm | 250mm | 300mm | |
| 640 Transmission Level Sensitivity 0.04 mm/m | x | x | x | x | |
| 640 Transmission Level Sensitivity 0.1 mm/m | x | x | x | x | |
| 640 Transmission Level Sensitivity 0.3 mm/m | x | x | x | x |
