HP-75C/D Processor Question

[Mike T.]

What processor was the HP-75C/D based on?

Since the HP75 supports IEEE floating point numbers I presume that it is has 64 bit registers unlike the 56 bit registers used the earlier machines.

The obvious candidate would be the Saturn processor, but this site says that the HP-71B was the first machine with the Saturn CPU and as this was released after the HP-75 this suggests that the HP75 used something else?

Just curious...