KL10-A/KL10-B differences

So I'm a little puzzled by something, and I was wondering if anyone here knows the answer. So early KL10's (KL10-A's, to be precise) only support a single DTE20, and no RH20's. Later ones supported up to 4 of the former, and up to 8 of the latter. I always supposed this to be part and parcel of the 'Model A/Model B' CPU difference, but no... EK-0KL10-02 Part 1 (no title, seems to be notes for F/S) pg. 9, says both KL10-A and KL10-B's are PA (DEC jargon for the Model A CPU - below), but the former has no RH20's, the latter does. (A note at the bottom of the page says that a PA is a 'Model A', and describes it as having "internal channels". The PV is a 'Model B' - extended addressing, larger ucode, faster clock.) So my new theory is that it's the MBox (either the backplane, the boards, or the wiring from it to connectors, etc) that is the difference between the KL10-A and the KL10-B. Can anyone confirm this and/or provide details of the differences? Pictures of the MBox in a KL10-A would be good, too. Noel