Walther WSR110
A late Walther WSR with small capacity, and in green. These are really fantastic machines, that can do anything you would expect from a mechanical calculator, do it well, and do it lightly, too. In addition, they have a geared sector mechanism, which means the input pins don't revolve as the machine is revolving. Backtransfer is provided, and the carriage has a life of its own when cleared - it jumps back to the left automarically when desired. Very well-thought out machines, that are a real pleasure to use.
Walther WSR160
And the slightly larger capacity machine, in gray, and with its plastified dust protection cover.
So what may not be too obvious about the controls? Backtransfer is clearly colour coded - pulling the red lever at the top an then the red lever to the left of the carriage will back-transfer the contents of the result register into the input. It will do that at any position of the carriage. The white lever on top just clears the input, without engaging backtransfer. It is easy to (also) accidentally pull the red lever, in which case the machine will be blocked, until the lever at the carriage is pulled and its contents back-transferrred, or the white button on the base behind the main crank is pressed, in which case the backtransfer mechanism will disengage without harm.
The other white button at the base, in front of the main crank, disengages the carriage lock mechanism, which makes it spring back to the left. There a tiny round selection lever at the back right end of the carriage which operates this spring mechanism - if it is in its normal position towards the back, the carriage spring mechanism operates on backtransfer and clearing the carriage, if it is to the front, the carriage stays put after these operations, and only the white button on the base can make it run back to the left.
Selection of the direction of rotation of the counter register is automatic on the first turn of the crank, but can be adapted manually if necessary (e.g. setting up a dividend from the input register with one positive turn - that's why the serrated wheels are on the result register, to encourage you to set up the dividend there ...)
Finally, there is a switch at the front of the carriage, which allows to clear the counter and result registers separately, th counter by pushing it to the left, the result by pushing it to the right.
The funniest story regarding these machines is one from a long time ago on ebay, when I just started collecting. There was an auction for one of these machines, and one of the pictures showed a veranda, which was chock-full of identical WSR160s. The explanation from the seller was that an older family member collected these, and distributed them to schools for educational purposes, on long term loan. However, when he died, his will specified that all the machines must be returned. So in the midst of the grief of their deceased family member, suddenly WSR160s started showing up on the doorstep, and the collection kept growing and growing all by itself, until there were over 50 machines! This is not one of those ...