SG 364a 1 1/2d Chestnut Watermark Royal Cypher "PENCF" Error positional marginal pair in Post Office fresh unmounted mint condition.

This Chestnut shade is of course commoner than the very scarce Red Brown shade variety many seem to persistently confuse it with.

It is well worth, or actually essential, that you have solid reference copies of these two basic shades to refer to when identifying these PENCF errors.