This is a guess, but Utah sold a "Add-A-Unit" set of assemblies in 1937.

The 500 watt transmitter was made of kit 1, 3 and 4. Kit 1 was the exciter
with 3 x 6L6 tubes and a rectifier. Kit 3 was an antenna coupling unit.
Kit 2 was the modulator with P-P 6L6 tubes. Kit 4 was the PA deck with push
pull Taylor T-55 final tubes and a pair of 866 tubes.

It was designed for a 1/2 rack.

If that is it, it is pretty rare.

73, Colin K7FM