Well, I've now tried all my CPUs (I have seven).

Three of them, including the one I started with, show the syndrome I
first posted:

+----------------
|   7..6..5..4..3..
| ?6 
| Failure.
+----------------

With at least one of them, the backpanel LEDs show 3, not 6, when this
happens (as implied by "3.."; presumably the "?6" indicates _which_
stage-3 test failed or some such).  I didn't check the backpanel value
from the others.  Two of these CPUs identify themselves as V1.3.

Two boards do

+----------------
|   7..6..
| ?3 
| Failure.
+----------------

with 6 displayed on backpanel LEDs.

One board passes POST and boots, apparently normally, but the boot
messages show "total memory = 5112 KB" (even less than I get with just
two boards).

The last board is the most interesting.  It passes POST and starts to
boot, but walks into the interrupt stack redzone very early in boot (I
posted a note about this, earlier; by now I've confirmed that it _is_
running into the redzone).  I'm still investigating that; if that board
can be made to work... :-)

However, I'm now tending to the opinion that the one person who told me
"I can confirm [] that the KA630 does run with three memory boards" is
misremembering (or, I suppose, deliberately trying to mislead me,
though I prefer not to suspect such things without solid evidence for
them).  Or, perhaps, had hardware or firmware with some kind of special
hack applied to support it....

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