As far as I know, there are about 120,000 examples of the New Zealand / Bahama Islands mule. That would equate to a normal maximum striking off a single die pair.
They were all discovered after being put into circulation.
Not 1960's :
British West Africa copper nickel Penny KM# 17 of Edward V111.
East Africa 1936H KM# 24, muled with British West Africa KM# 16
Mintage unknown, but very scarce.