1971 British Architecture 3p. Lemon omitted, Pierron GB1780M Ca group, S.G.890a group,

with just a sliver of the yellow printing remaining, fresh, original gum. VERY RARE. (Pierron and S.G. both unpriced)

In the Pierron handbook one used example of the total omission is recorded. This example would not seem to be a downward shift of the yellow printing as there does not appear to be any yellow ink in the green panel below - however, it remains a possibility