This intrigues me, as I've heard about it a couple of times now. I can't imagine any circumstance where either type of chassis could be anything but flat and the bearing perpendicular to it, so it must be the armboard. Shouldn't be too hard to remove it and check if it's true with a straight edge on a flat surface.
Construction is as above, with two layers of 9mm birch ply glued together with the mounting bolts through the bottom layer only and only the one nearest the platter having a nut fitted. The staples were just to hold the layers together during assembly. Dimensions are available here somewhere but could easily be taken from your existing board.
The Rega arms need a 23mm diameter hole 221.7mm from the center spindle. The location of the hole on the 221.7mm arc is determined by the clearance between the counterweight stub and the rear of the lid, but i'm sure Svend would be happy to measure his when he swaps his arms over!
Time flies like an arrow.
Fruit flies like a banana.