Interesting. There clearly must have been a transition period from Simplex to Campagnolo gear and dropouts. I have
s/n 104060 which has Simplex dropouts. It came to me with a Simplex Juy 61 rear derailleur and typical (I think) metal matchbox Simplex front derailleur. When I first got it, a bit of searching suggested it was late 50s. Then I saw Dale Brown's
serial number registry which clearly suggests early- to mid-60s.