From MetroAmp Wiki
Mercury Magnetics O100JM-SL output transformer. A self lead version their O100JM used in 100 watt Marshall amplifiers.
Technical note:
As an accurate reproduction of the early Dagnall C1998, this self lead transformer has the solid copper leads from each secondary winding extended fully.
From Paul @ MM:
This means that the wires coming out of the yellow tube have to be tied/soldered together. The wires coming out of the black tube need to be tied/soldered together and the wires coming out of the green tube need to be wired together.
Even if the 16 ohm tap (wire coming out of the green tube) is NOT being used in the amps circuit, the two wires MUST be tied together for the OT to work properly.