Colored shapes on GB Machin stamps

Collectors are often puzzled by odd markings on Great Britain stamps, and the question is quite often asked by email or on philatelic forums.   Sometimes it is postmarks, other times odd marks such as these:

These pale blue or white opaque phosphorised dots are part of the address coding to enable mechanised sorting of the mail. Previously they were transparent, and later they were replaced with red or red-orange bar-codes.

This has been discussed on the Stampboards forum - come along and join in the debate and show your examples on stamps or covers!

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