The board had a variety of commercial overprint patterns; the ones of interest here are the EEB/C patterns. The main EEB/C patterns are:
- Small letters (1.6mm high), widely spaced. These appear on sideways watermarked stamps from SG 488a to 613ab.
- Medium letters (2.0-2.2mm high), tightly spaced. These appear on stamps with normal and sideways watermarks from SG 518a to 613ab, and the unwatermarked SG 727.
- Large letters (2.8mm high), tightly spaced. These appear on stamps with normal watermarks (SG 579 and 613a) and the unwatermarked SG 726, 727, and 736.
Based on receipts I have, the "C" could refer to the Chilterns sub-area (small letters pattern on SG 506a, dated 14 September 1954) or Cambridge (medium letters pattern on 573 or 573a, dated 24 May 1966). It seems that the EEB/C patterns may have been used interchangeably with the more common E.E.B. patterns--in addition to the receipt from May 1966 just mentioned I have an E.E.B. pattern on SG 573 to the same address dated 25 May 1965.
by Michael Behm
References: Electricity Act, 1947