Unidentified Component in an AC motor speed drive unit
Probably it's not a standard component.
It just a (custom) sub-circuit probably in thick-film hybrid technology or just on a small PCB potted with resin inside a plastic box.

One reason to do this might be 


to make it more difficult to reverse engineer (what Jack Creasey noted) but there are also other reasons, e.g. 
to make crtitical parts mechanically or chemically more robust, 
to have better thermal coupling, 
to produce the subcircuit common in various designs in higher quantity and only vary a few external components for each versions etc.