It is true, this isn't a screw in cap. As you say PJ, it takes a turn of the key to control the recessed pins.
The recessed area is not geometrically a true square, but it does have four sides. There are curves to the sides and the length of the sides differ.
Here is a fair image of this area:
Measurements of the cap (as best as I can figure due to its odd shape recess) are:
Width at the widest (in the center) = 3 1/4"
Length (front to back) = 3 3/4"
Width of the actual center filler hole = 2 1/4"
Thanks PJ