Hi D F ALLEN refers to this legend reading in Henry II Cross-crosslets and regards it as standing for Senior or Secundus Types for class B, E and F,pg XXIV and XXXI AND XXXII. mints recorded Exeter (B or C) no 273, Ilchester (B3) no 278, Ipswich (B3) 298 London (E) 509 pl 22 17 ( different obverse die to yours) Your coin appears to read N:CA which could indicate Carlisle or Canterbury both of which struck in class E