Hi Pete
Fancy you noticing this - I don't recall seeing any other reference to it.
I have had a quick browse and can offer some more evidence to help with your musings. Short version: I have some planes with this feature and some without, from Bristol makers and from others.
First off, these are all from Gardner and show the second angle on the escapement as you describe:
This Greenslade has it, though somewhat crudely cut: (this plane has been repaired)
So does this tidier Greenslade:
This one by Gardner late Holbrook has two angles, though the difference is slight:
Those are all local. Turning further afield, this bead by John Elsworth of Glasgow has the same feature
And this one from Marshall of Glasgow:
It's not just the Scots: this slipped bead is by Griffiths of Norwich and has two angles:
as does this one by Preston:
(This may have been Mr Ramsay's favourite plane...)
I have other similar profile planes which have just one square cut. Makers must have thought it worth the extra effort - perhaps it was a feature found only on the 'best' lines, not on the ordinary ones. I don't know what made it worth having; I'll have to do some experiments and see if I can observe any difference in use.