Constant varieties exist but one needs a specialized guide.Older Suarnet
catalogs show some varieties for the Yt.37 1870.The colony stamp (Yt.12 Imperf.) was from the same plates re-printed in 1871.The plates were by this time worn out.Plate reconstruction of Yt.37 has been achieved.
Philatelists décades ago have found similarities with constant galvano flaws with the 20c black of 1849.It was assumed that certain galvanos were used for Yt.37.thus we can assume for Yt.12(Colonies) also.
It is difficult to say if this is a constant variety or a galvano flaw which was replaced quickly once discovered during the printing.
In any case it is an interesting variety.
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Below half of panel D4 (150 stamps) for the 20c black 1849 & 20c blue of 1870.The numbers refer to the position in the sheet.
Most of the numbers refer to breaks in the outside frame.The depiction of the breaks and their lengths are quiet precise in the reproduction.
