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in contrast to the other examples I have of this type, both imperf and perf, and also in the values with different colours, the background lines are oblique. In all the others, the backgrounds are horizontal/vertical cross hatching
I'm a bit confused by the question, as this design is found on Belgium 1 and 2, the 10c and this 20c. Both were imperfs. The design changes for subsequent stamps.
This design has scrollwork running down the background behind the portrait. It is more prominent on the right side (his left) than the left side. The background lines should look like diagonal lines, running from top right to bottom left.
I don't have any of my resources at hand to see if this is a forgery, but the overall appearance is generally what I would expect to see.