As I understand it (and I could dead wrong), the 323D replaces the 320C directly, whilst the 320D is more a replacement of the 319C.
But that doesn't make perfect sense to me, so I'm only offering my thoughts -- not concrete. I'll try to verify, but it's really busy for the Cat marketing guys right now...
319c??? You Mean the 318c???
This is all getting confusing. They should have just stuck with the old numbers.
319c??? You Mean the 318c???
This is all getting confusing. They should have just stuck with the old numbers.