(The plate position is ) 157UR92, but there's a twist.  Only a small part of the markings produced by the misplaced entry appear on the proof.  

The attached image highlights the marks on the proof. The marks are encircled and a line leads from the circle or ellipse to the edge of the stamp.

For the most part, the marks on the proof are the strongest ones on the stamp.  For example, only three segments of the doubled frame are visible in the bottom margin on the proof, but they are the strongest parts of that doubled frame.  I can understand why Marler wouldn't have bothered with this - the marks on the proof are nowhere near as impressive as those on the stamps.

I feel confident that 157UR92 is the position of the misplaced entry, given:

• The correlation between the marks on the proof and the stamps
• The identification of the design type which is why I examined plates 157 and 158 for the variety
• The straight edge at the bottom of the stamp