I wondering whether or not what you're seeing is the evolution of Fitz manufacturing, with differences between earlier manufactured grips and other grips throughout the life of the grips.
Linde, in your post back in 6/23/13, you posted this pic that shows the flat shelf with a metal disc added, which to me looks to be an improvement over the original inletting made by Fitz, which is a "M" marked grip with just the flat shelf.
This is what the inletting looks like on the Gunfighter grips I bought in 2008, which are the most recent and last ones made... note that the inletting style is what you find on most aftermarket grips, with what S&W calls the "stock circle" incorporated into the wood:
