Because the 67 and earlier cranks had the smaller register. I am not sure about 68 and 69 cranks , but my 273 had the smaller register in it. I found out the hard way, after building a 727 and getting a torque converter, then realizing the delema.
So it must be that the '67 and earlier 904 must have a smaller center button on the converter and the '68 and newer 904 must have the same size center button as the 727? This must be why it worked on my '69 I'm guessing.
