A later revision (-002) of that manual updates the
Figure 1-6 on page
I/1-9 and explicitly labels the heads upper (1) and lower (0) while
retaining the text throughout that refers to them as the opposite of
that. Interesting and confusing!
I guess you'll have to measure to find out.
I wonder if the heads were originally refered to as the 'up' and 'down'
head. That would have been conventional at one time and refers to the
direction that the heads face. So the 'up' head is the lower one, the
'down# haed is the upper one.
If this got edited by somebody unfamiliar with the terms, the 'up' head
may well have become the 'upper' head, hence the confusion
-tony