Not thick Mike, there's a slim chance you have something very unusual. It's definitely fitted, it has a vac pipe. The most likely explanation would just be that it looks dusty, but enlarging the shot, there do appear to be areas of light grey patch painting on the end - look particularly at the 'grid' pattern on the LH panel.
I've always understood that the far North went fully fitted in 1977, at the same time as the Southern, yet on the cover of a book I have (Tom Noble's Diesels on the Regions, ScR), there is another shot, also very coincidentally taken at Brora, of a 26 with a 16 tonner that is even more clearly grey. That one is dated March '83, which is very late for any unfitted 16 tonner to be away from the areas in which they were concentrated at the end. Not impossible for it to have strayed of course, if it turned up at Inverness then no doubt it would have been sent on, possibly under some local authority or on the basis of 'we've dealt with these bl**dy things for long enough, we do know what to do with them'.
I'm far from convinced, but open minded - it's just possible that a few of these wagons were vac fitted at a very late date without being repainted, in the same way as many 21T hoppers were, but definitive evidence is going to be very elusive, methinks.