Proportional Controller ?

Maybe someone can Identify what I have.

I picked up (2) devices that look exactly like a solid state relay.
But the are not, they say "proportional controller" on them.

Made by Douglas Randall, model # R25A
On one side, terminals 1 and 2 say AC Load - 25 amp
The other side, terminals 3 and 4 say Resistance.

I did a search that turned up little. I have some assumptions to what
they do but maybe someone who actually knows could explain it and give
an illustration on a typical application.

I'd rather that than connect it up and do a little experimenting.

Thanks,
Chris L