Folkard states:
State 1 – East side weak near top, imp unbroken; West side weak.
State 2 - East and west sides weaker; West network slightly thinned; Postage slightly thinned; Value thinned; Engraving to head relatively strong.
State 3 – West side stronger; C thinner; Top line thicker; N side NW sq double; Engraving to head weak.
The only stamp I have (BL) which lacks the letter squares, shows some features consistent with state 3 of this lettering.
Imprimatur
173 BL