Tim,
My understanding is that for low compression engines, lead shouldn't make any real difference (it helps stop pinking). In any event, as RF says, lead additives came in during the 1920s.
This may of interest more generally: http://www.quillertriumph.co.uk/Quiller/Unleaded/federation%20info.htm
Tim, which octane rating of unleaded do you use? have you tried high and low?