Jennings Electronic Industries reverb units
Early JEI reverb units (that is to say pre-1973) are extremely thin on the ground these days, probably a result of a combination of factors: hardiness, quality, and small production numbers.
But at least two post-1973 units in new-style livery survive, one pictured below. Their electronics probably have little in common with earlier versions given the greater sophistication of the circuit - a "blend" control is provided, for instance, along with high and low gain inputs.
Small advert from a Jennings flyer promoting pedals and gizmos. Presumably (given the stated dimensions) a single spring tank inside. Quite how long the batteries will have lasted is anyone's guess.
Part of a note in Beat Instrumental magazine, August 1971, on Jennings equipment at the Associated Musical Instrument Industries fair (London, Russell Hotel). The new reverb unit, priced at £22, is the RV1 picture in the notice below.
The reverb unit mentioned above, styling to match the early PA units. The PA100 pictured has reverb incorporated, one of the few units to have been kitted out in this way.
A Jennings reverb unit from c. 1973
The unit's outer dimensions are: 13 3/4" wide; 7" deep; and 2 3/4" tall. The power inlet on the back panel is a mini Bulgin.