Marconi MkIII Focus supply unit Type 2137 editions A to H.
More thoughts on the MkIII PSU. As the PSU was developed from the late model Mkii PSU with a requirement for 75mA. of focus current for the smaller 3" tube, the very similar PSU in Mkiii mode was satisfactory for the Mkiii camera with a 3" tube, but not the 4.5" tube. Marconi made an additional PSU to cope with the 140mA needed by the 4.5" tube focus coil. This PSU, Type 2137 mobile, or Type 1881 19" rack unit, is quite rare.

The Mobile case version could hold up to 4 focus supply units in 117v or 200/246v inputs.