Um, Peco boards have symmetrical cut outs (equidistant from the midline), Cambo boards' cut outs are asymmetrical.
KEH is so-so on boards. They've sold me a non-Graphic board that was considerably the wrong size as a Graphic board, they've sent me a 4x5 Graphic board when I ordered a 2x3er and once sold me a Cambo 4x5 board milled down to fit a 2x3 Cambo as a real 2x3 Cambo board. Re this last, the board fit but the light trap ridges weren't there.
That said, KEH seems to identify 4x5 Cambo boards correctly and they're usually the low cost vendor. At the moment, though, KEH has low stock. You might want to ask mpex.com what they have on hand.
The Super Cambo ("SC") was sold in four sizes. SC-1 for 2x3, SC-2 for 4x5, SC-3 for 5x7 and SC-4 for 8x10. -2, -3, and -4 all use the same front standard, all sizes use a 1" x 1" square rail; the rail's length varies with the camera's size. Later 4x5 Cambos all accept the 4x5 SC board (6 3/8" square, with rounded corners) and 4x5 SC standards accept later boards. 2x3 SC-1 standards accept 2x3 SF and Ultima boards.
You might want to look for boards on ebay.co.uk and ebay.de but I'm not sure that shipping costs will be lower from the UK or Germany than from the US.