Practise keys were used.... Some practise keys were manual only while others, known as buzzers, had a feedback sound to assist the student.
Key Dummy Signallers Mk II
A dummy practise key made of brass and wood. Brass part is stamped "Key Dummy Signallers MkII, DOM LOOSE-LEAF Co Ltd, No 823"
Practise Buzzer - Signal R-68
A practise buzzer/key made by Signal Electric Mfg. Co. - Menominee, Mich and C Broad-Arrow stamped. Instructions on bottom show how to use the buzzer as a single or with another unit.