As far as I remember (and it was 20 years ago that I was running out of Shaftesbury), they’re both “Unsuitable for HGV” but from opposite directions, if that makes sense? So the Melbury Abbas route has a blue sign next to it at the other end, while the A350 doesn’t - or certainly didn’t.
I once got stuck on the hill on the narrow bit of the Melbury Abbas route, going the “wrong” direction - at rush hour on a Monday morning. Bring back dodgy twin-splitters in Ivecos being driven by 24 year old girls with bugger all experience! 