Just measured it now. The upper holes c to c are 2 inches apart . From the upper left hole to the bottom enlarged hole [ 2 shots] is 1 5/8ths and from the upper right to those two is 1 inch So,
2 inches x 1 5/8ths x 1 inch
This .40 that I built about 10 years ago , fits me perfectly for summer shooting. When I bring it up, the sights are on. I used to want more weight up front and was sorry I used an A weight swamped barrel at first, but now that I'm officially in my senior years , I've come to really like it. The Davis triggers are set very light, and that is a tremendous help.