It is difficult to date Optimus stoves, as unlike Primus, the manufacturer did not date-code their stoves. However, by studying the dated Catalogues and Brochures contained within the CCS Reference Section, it is possible to follow small design changes which occurred during the production period of stoves. I have been looking at the positioning of the air-release valve on Optimus No45/48 stoves, as an aid to dating particular examples, and here are some observations I have made: With stoves things are never simple. Over its lifetime the air-release screw on the Optimus 45 would seem to have moved around a bit! 1. From the top of the tank, in the 1920 and 1929 catalogues. 2. To the top of the fuel filler cap in the 1932, 1933, and 1939 catalogues. 3. To the side of the filler upstand in the 1957/8 catalogue. 4. To the top of the fuel filler cap in the 1959 catalogue. 5. Finally, back to the side of the filler upstand in the 1972, 1975, 1984, 1985 and 1989/90 catalogues and brochures". I would welcome any comments or observations on the above, and will update the list, as and when additional dating information becomes available. Best Regards, Kerophile.