I believe that Irvin uses a Wise 2086 tension head with the Gamma adapter (same as I use). It brings the top of the diabolo and the string gripper level with the grommets, which is different from most other machines (which drop the tension head to allow 360 degree rotation).
In that configuration, the string's path is essentially the same whether it goes over or under the frame. Over is usually more convenient for the stringer.
USRSA MRT - Wilson Baiardo
Wilson RF97 Autograph - ALU Power Spin/ALU Power Feel