The Radius Arms were very straightforward and per the manual, which indicates the dimpled surfaces of the arms face outwards, the chassis end of the arm being mounted on the lower hole in the chassis (for R-specification cars).
The only slight difficulty was that the DeDion bracket was very tight for the Radius Arm to be fitted. A block of wood was used to apply sufficient pressure (without damage) to the brackets and open them the tiny amount required to allow the Radius Arm to slide into position.