5¢ Blue 10UR100
On stamps with a large bottom margin there are two guide dots near the lower right corner. There is also a guide dot at the right centre. There is a guide line which traverses the lower guide dot which will differentiate 10UR100 from 9UR100.
Research notes:
This item was not listed in his book, but was present in Marler's personal collection with the suggested plate position annotated.