Duo-color (blue and black) trial proofs in large and small
varieties for the two stamp designs in Portugal's Porto
refinery set. Comparable two-color proofs exist for
these stamps in several other colors. The upper
design depicts catalytic cracking towers; the
lower one shows a distillation plant. Both
designs were adopted for use but were
executed in different colors (see
below for two examples).

© Peter A. Scholle, 2000

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