Porlock skeleton, 1926

I have recently acquired this copy of the "PORLOCK / TAUNTON SOMT" skeleton, used on 11th May 1925 at the end of the known period of use.

When I measure it, I find a diameter of 29mm rather than the 30mm in "Skeleton Postmarks of England & Wales", originally by James Mackay but now maintained by Patrick Awcock.  I've raised this with Patrick and will see what he says.  Here is an enlarged and cleaned up image of the cancel above.