I'm going with two as the 1940 parts list clearly states that quantity (pair...Qty.1)...
The questions now are how thick were the plates and were they spring steel...By gently applying some pressure to the ones I have I can test their ability to return to the original shape...If they are spring steel that will be obvious..However, these didn't come with the horn and look rather too good to be originals so I'm not keen to use these as definitive proof of anything.....So, the two questions remain...Ian