It's the small stone between the the ante-chamber stone and the doorstone
Are you referring to the packing stone at the top of the 'buttress' George?
Yes
Except I wouldn't describe it as a buttress .
Not originally no.
If it was a buttress why the padstone and why isn't at more functional acute angle to actually act like a buttress ? The more obvious simpler explanation is that it is what it appears to be , an ante chamber stone .