Click to enlarge
What struck me was the appearance of Stone 40 (a) in the middle foreground of the older picture and how it has disappeared into the grass today.
It gives a hint of a mortice hole half showing but it is probably just a trick of the light.
Cleal et al doesn't seem to have an entry for Stone 40 (a) but luckily there is a website that does:
As you know better than most, there hasn't been a lot of wear-and-tear on the stones since c.1960, (the lichens are another story!) While it might be fun to imagine there's a mortise on BS-40 — given its position — I believe it's a trick of the light.
If anything, it should have a tenon.
Personally, I think -150 was, back in the day, used in the forecourt as a bluestone entrance lintel and -36 was set up behind the Great Trilithon.
Conversely, like so many others, -40's been chipped at and wacked pretty hard over the centuries, so perhaps there was a tenon on it originally.