Yeah cool Mick. Great pick up.
Prototype would be '78 I guess. Though as the pedal is stamped "Patent Pending"you could assume it was made after the patent application was lodged in early Jan '79.
And as no-one has ever seen an alloy Kingsbury it'd have to be one of very few made early on.
So my tip is late 78/early 79. Extremely cool pedals.
Great condition too - those ends are awesome.