I’ve found a photo of an SD4 with test trailer which is identical to the previous picture and indeed the text with it says it would have been a special types before the length limits change in 1962. My list of C&U Regs has no changes for 1962 so not sure how that stands unless its a mistake and should be 1964. It does seem to be another Dyson trailer possibly even the one that went to GKN in the other pic. What was needed for ‘Special Types’ identification in those days? Franky.
