I believe it is a Marconi device, a large, heavy, complex and undoubtedly expensive bit of kit. A fairly good example of '50's defence over-engineering with its high resolution film scale tuning drive and quite complex saturable reactor FM modulator. I have heard it described as "a TF144H designed by a committee with someone else's cheque-book"
Same size (i.e.6U) front panel as the TF144H but probably double the weight and chassis depth. Nonetheless, a good attenuator- something that sorts the real McCoy from the toys in the SG world. That looks like it has had the PSU mods (clue: voltage selector panel with two, rather than one primary change-over toggle for series/parallel 100/200V ranging) that adds a second mains transformer and additional stabilised HT/LT supplies.
How's your weight-training?
Edit- actually 8U high!