"Hand Yarn Twist Tester", made in China ...
https://www.ebay.com.au/itm/325117213756?
https://www.ebay.com.au/itm/164749199309?
https://textilelearner.net/twist-measurement-of-yarn/
The OP instrument does not test tension, it measures the small change in length of a spun yarn as it is twisted. Procedure: The spindle counter dials are set to zero. The yarn to be tested is clamped at the spindle and at the length gauge, with a weight beyond the length gauge, to apply a constant tension. The spindle is then turned to remove the twist, which lengthens the yarn, but turning is continued until the length gauge shows the yarn length has reduced again to the same reading as initially set, (but with the twist now reversed). Half that spindle turns counter reading is the number of twists per length tested. No magnifying glass is needed to examine the yarn to determine the average point of zero twist at different positions along the sample.