This is one of the TG12413 Limiter modules from the TG12410 transfer console.

Based on the limiter/compressor from the TG mixing console they are self contained, requiring only a power supply, meter and input/output transformers to become a stand-alone unit. A pair may be ganged together for stereo operation.

They have the disadvantage of not having an input gain control, which can make them awkward to use.