Images of hidden codes on Security Machins

My copies of the Machin booklets have arrived. Here are two images of the hidden codes. Above is "FOYAL" with the "F" circled. This is the large second-class stamp from a booklet of four. Below is "MTIL" with the "T" circled. This is the standard second-class stamp from a booklet of 12.


The gold color of the first-class stamps is harder to scan, but I'll give it a try and post them if I can get good results.

When doing these scans, I noticed for the first time that the letters of the overprint are smaller on the large-size stamp than they are on the standard-size stamp. You can see that in these images. I wonder why that is.

--Larry