To my knowledge the “oval holes” are actually meant to be double-D holes. They were originally used in the Eurocard rack-cases to take plastic or metal screw holders which would snap into them. If so they would have nothing to do with non-standard panel widths or different modular standards. The distance of the mounting holes is clearly defined, the tolerances of panel widths just depend on the manufacturers capability. -- [sdiy] Parts going out of module panel, Matthias Herrmann, 9 May 2018.