This transformer mounts inside the HE440 control panel. The transformer output is rated at 24 vac at 20 watts. The mounting holes are spaced 2-7/8".
To test this transformer, connect 120 vac to the primary wires, the white and black wires. The voltage on the secondary wires, blue and yellow, should measure at 24 vac.
For wiring info, see the schematic on page 11 of the HE440 Product Data manual.