Reading P7sa back to a C1

[BR60103]

There were two types of reversing controls: rotary and lever. The lever swung in a big arc -- often above the engineer --along a curved segment with notches and had a latch that fitted into the notches to keep it in position.
The other type had to be turned. The drawing and photo suggest a wheel; when I was in Flying Scotsman it was a crank on a vertical screw rod with a pointer on it.
Either type shifted the valve gear to adjust the point at which steam was admitted to the cylinders. At the extremes, steam would be admitted for much of the stroke, reducing to no admittance (and no valve movement) in the middle. Note that most model steam locomotives have their valve gear set in the centre position.
The rotary handle gave a fine adjustment to the "cut off" but was miserable in a switching job when every direction change meant you had to spin the handle for ages.
(sorry: too much info?)