We've discussed this thing before, but without the benefit of such a terrific picture.
That is the "Practical Wireless" design of (about) 1967/8, almost exactly as the magazine article intended, cabinet and all! This one has four pedals and the original design (I'm sure) had two, but allowed for builder "improvements"!!! Like the two "extra" strings mentioned in the letter.
Could be worth a pound or two; I reckon it'll burn for much longer than a banjo!