5¢ Blue 7LR10
On stamps with a large top margin there is a triangle of three guide dots at the far upper left. In his collection, Marler had an additional example with an even larger top margin which also showed a large guide dot in the far upper right. It is this dot that he matched with 7UR100.
Research notes:
This item was not listed in his book, but was present in Marler's personal collection with the suggested plate position annotated.