ITV Channel 11 Test Transmitter.

[Brianc]

I would suggest that the smaller box is the modulator, the third output, which goes to the front panel, being a monitor output . The larger box is the filter as the output goes directly to the rear of the module and I assume that there is a coaxial connector under the "tongue" that can be seen to the rear of the box - say!
Cheers
Brian