Only to add that in my experience all S1 headlamp surrounds are made of brass and bright chrome plated, held in place by studding brazed to the rear faces – unlike later ones made of aluminium with visible front fitting screws.
Having looked carefully at the 1966 photo posted by Paul, I am not so sure that it is clearly flat. It looks to me to have a recessed edge, but with plexiglass covers over the whole thing. Does anyone know how the covers might have been fitted? There is no sign of any clips.
Sorry, I don’t have any spares.
Colin