All of the Hemi engines they built probably had a mechanical fuel injection pump driven by the gear drive. The hub for driving the pump was bolted onto the cam gear right where you would put a cam button. Iirc there were shims between that hub and the round cover to control cam thrust. Therefore a traditional cam button was not only impossible but also unneeded. The pump drive and gear drive cover served the same purpose.
I have not had one of these apart for 45 years so I may have missed some detail but you get the drift. Injected engines would not/could not use a traditional cam button and didn't need one. Everybody else does.