Interesting ... I wonder what the red and white road means on the legend?
Single-track. See OS mapping, for instance.
Technically, "narrow road with passing places" according to the legend.
It would also be interesting to know whether other short A/B roads are numbered on this Barts map - if the A951 is unusual in not being numbered it may suggest that the number has been removed without updating the shading of the road itself.
Mmm, it's a very late Bart's! Very hit and miss - the A951 isn't at all unusual.
All in all, I would take this particular mapping evidence with a pinch of salt, sorry!
Yes, it was just interesting to note the extended A951 on it, but as I remarked, by the date of the map it had clearly been truncated for at least 6 years.
Edit, just looking again at Stevens earlier extracts, the A951 is shown as S1 in 71, S2 in 72 and truncated without the kink at Aviemore by 74. This may, of course, be related to the obvious changes to the mapping styles, but it may just give us a chronology.
There is still a style for "narrow road with passing places" on the 1972 map which gets used extensively in the Highlands, if that helps.