Something which occurs to me which may be of help with correlation as more dated examples come to light: If the parts lists are to be believed, the air release screw fitted to the No.21 was shared with the Radius No.20 and Radius No.17 (and other much less common stoves) and even the filler cap seems to have been shared with the No.20 later on. In the 1932 Radius catalogue: The No.17 air release is part 21/43, the filler cap is 21/45 The No.20 air release is part 21/43, the filler cap 20/45 (specific to the No.20) The No.21 air release is part 21/43, the filler cap is 21/45 In the 1937 Radius catalogue: The No.17 air release is part 21/43, the filler cap is 21/45 The No.20 air release is part 21/43, the filler cap 20/45 (specific to the No.20) The No.21 air release is part 21/43, the filler cap is 21/45 In the supposed 1950s Radius catalogue: The No.17 air release is part 21/43, the filler cap is 20/47 The No.20 air release is part 21/43, the filler cap 20/47 The No.21 air release is part 21/43, the filler cap is 20/47 ....suggesting rationalisation of production by the 1950s, doing away with the sunken/recessed filler on the No.21 and using the same cap (and even neck?) as the no.17/No.20 as a result. Given the part number of the air release screw is consistent across all three stoves, can we assume that the size and knurling changed simultaneously across the range? Despite the different part number for the No.20 filler cap in the 1930s catalogues, was the lettering style and knurling similar, or did it differ? I haven't the time at the moment to study the reference gallery to confirm, but the nights are drawing in!
