Following on from my previous post i thought i would check out ebay as to their availability. These little things seem to be quite rare in the cylindrical shape in the photos. There are plenty of the more modern oblong shape but these are not really “period”.
Found some for sale at https://www.britishpipeclamps.co.uk/product/radio-suppressor/
The Haynes manual has a comprehensive section about interference suppression in the IGNITION chapter.