If you have magneto ignition on your ’47 you really should not have ‘radio supressed’ plug caps. Much better to use a plain brass connector or the QD Scott ones. If you buy a pair of those note that ‘modern’ spark plug tops are threaded 4mm, while older plugs are threaded 3BA, so the bronze balls will have to be threaded accordingly. On my vintage TT Scotts I have a pair of ‘period’ Lodge H1 (18mm) plugs in for display purposes only, and for actual running I use the new ‘repro’ KLG ML50 plugs sold by the Green Spark Plug Company, and I have to swap from 3BA to 4mm threaded ball connectors. If you go onto Tim Green’s website you will find lots of different plug caps and fittings.