The Jennings Electronic Industries Phaser was first brought to market in the Spring of 1974:

"Beat Instrumental" magazine, May 1974: a note on the Jennings Electronic Industries (JEI) showing at the Frankfurt Musikmesse in March of that year.

In December 1974, the pedal cost £32.40.

The pedal was evidently not an "in-house" design, however - at least not in its entirety. Its circuit actually derives from a circuit first published in "Practical Electronics" magazine, September (not October) 1973 - see this thread. An update does figure though in the October issue.

A full copy of "Practical Electronics" September 1973 can be found here. The circuit is described on pages 772-774.

The JEI Phaser is quite unlike modern phasers, its swoosh and sweep being calmer and more musical. Much as with a wah, the pedal gives one the ability to regulate the effect by foot. The gain generated by the circuit is immense.

Below, some fairly general (for the time being) pictures of a unit with original box. The casing, by the way, is not metal but tough plastic.