Yes, I was told the original box at Grosmont had been nearer the junction with the national network, and that NYMR had decided that site was too far from the loco shed the other side of the short tunnel.
What about Hythe & Dymchurch? Is that line excluded because it wasn't part of the national network (but neither was Bala Lake) ?
Are the frames on the Speyside's Boat of Garten boxes replacements?
I think the definition it is fairly arbitrary in requiring continued use of a pre-closure frame, since preserved lines have completely different traffic patterns and necessitating extensive track layout and signalling changes often amounting to a resignalling, and it is not unusual to replace the frame in a resignalling (ie BR might well have replaced the frame if they were making the same changes).
Is Horsted Keynes really original, given that the lever frame support timbers had to be replaced, the levers were removed for a major resignalling leaving the layout being worked for several years by switches on the back wall, before being put back onto levers? Would it be less original if they had used a different frame?
The trouble with looking for an "original" box is that woodwork does need to be repaired, moving parts renewed, changes made to meet traffic demands, etc, all of which were also done as required by the original operators - that's why in some cases the box was on its second frame at closure. It's like the broom that has had two new heads and three new handles, but it's still the same old broom.