Cambrian Airways
Cambrian Airways took over British European Airways Irish Sea Service in 1963, and unofficially carried some airway letters. The airline received official authorisation from the Post Office to carry airway letters over its routes from 2nd December 1963.
1st April 1963. Provisional Issue.
Photo-engraved by Harrison & Sons. Sheets of 25 (5 rows of 5). Perf. 13. BEA sixth issue with 'B.E.A.' on postmark crossed out and replaced by 'Cambrian'.
Orange-red and black

2nd December 1964. First Issue.

Photo-engraved by Harrison & Sons. Sheets of 10 (2 rows of 5). Rouletted, imperforate margins.
Orange-red and black.

Ultramarine and black.

Light green and black.

Cambrian Airway Letter Service Air Mail News, Volume 5, p. 88 (1964)
The Story of the Airway Letter Service 1933-1988 D. Wilson, Air Mail News June 1994