1. This rule has the same sale set as the 10 inch equivalent (1/98 Elektro)
2. Whilst one of the advantages of 20 inch rules ,compared to 10 inch rules, is increased accuracy, this could scarcely have been much of an advantage in this case since the errors in fabrication of the cables would have been larger than the 1:1000 accuracy of a 10 inch slide rule or the 1:2000 accuracy of a 20 inch slide rule.
3. There are windows on the back with hair lines on the stock for sin and tan (left) and log (right). The sine scale goes from 35' to 90° and the tan scale from 35' to 45°.

Front view

Detail - front left

Detail - front right