The two MB-51 8 wheel tipper models you show without the Pointer labels are most likely to have come from the Super Action Play Set series, P-3 Load-A -Vator. I tried to find a photo of my set but couldn't to show you.
All the sets I've seen and the one I have, this model has no side labels.
I think it's listed as such in Charlie Macks book and is also mentioned in the Matchbox UK magazines by Ray Bush.