Harmonic balancer bolt missing need help

My 64 slant has no bolt nor a threaded hole in the crank as best I can tell. The engine casting number is listed for 1966 according to one source. A 74 engine I have does have a bolt, as I recall turning it over with a socket, but might have been added by the prior owner. A BB bolt fitting would not be a surprise since I understand the slant lower end shares some parts with BB engines, which is one reason they are so strong.