Red test button
Identical in function to the Chilton ELCB, these were the other main type available prior to 1981. They are made from black plastic and have a small round red test button.
The images here show the front, front with covers removed, and the rear panel.
Supply wires enter at the top and leave at the bottom. The Frame and Earth wires both connect to terminals under the lower cover.
These devices are obsolete, over 30 years old and should be replaced.