Thanks for all the replies, folks.

Yes, just been looking at lens boards on eBay, and Sinar look to be the same - one in particular with good photos showed that it has the same lip on the inside to slot into the recess in the front standard, and even has the same Hammerite-style grey paint finish on the outside. Given that this Sinar, and my MPP panels, all have a silvery foil name sticker, my guess is that they were made by the same company back in the day. I think the nominal size is probably 5.5 inches.

I just did some measuring using a lens board with no lens, with the standards set as close as they'll go. From the outer surface of the board to the ground glass, I got 87mm. I think that means I'm in the clear for using a 90mm. Even if it turns out that infinity focus can't be acheived, I should be able to get enough sharpness by stopping down. If a recessed board really is needed, it's good to know that there might be an option with the Sinar ones - I don't think I've ever seen a recessed baord for the MPP monorail. If that can't be found, there is always the option of making one from from a flat board.