These items are electonic substitution boxes.
Sounds pretty technical, but they are basically boxes with parts.

If the repairman suspects a paticular part of a radio to be defective
they can quickly and easily substitute the suspected bad part with
the identical good part kept within these boxes. That way they
can readily identify a bad part.

The item on the far Left in the thumbnail is a transistor tester
by Micronta (Radio Shack). It works and we also have
the owners manual.