Security Machins overprint

I finally had a few minutes today to play with the security Machins. After scanning one and a few minutes of playing around in Photoshop, I got this image that clearly shows the overprint and the slits. (Click on the image for a slightly larger version.)

Notice how the overprint follows the portrait's contour. This is most noticeable on the Queen's shoulder and on her hair at lower right. The "Y" on the hair is distorted to look almost like a large "V". I wonder if this is responsible for the overprint being nearly invisible in ordinary light.

--Larry

PS For anyone who would like to duplicate this in Photoshop or other image manipulation program, all I did was lower the brightness and increase the contrast. In Photoshop, I set the brightness to -60 and the contrast to +60.