[TenTec] Corsair (Model 560) frequency display derivation

[Keith Johnson]

On page 3-27 of the Corsair manual (Model 560) under the column labeled
DISPLAY (near the middle of the page) it states that the display will
read -1.000 when the VFO jumper is 'removed' and the rig is set for the
3.5 Mhz band (80 meters).  What does the value -1.000 represent?  In
other words HOW does the frequency display derive a displayed frequency
on the 80 meter band and how does the -1.000 value play a part in this? 
I believe I understand the other values that exist under the DISPLAY
column in terms of why they are used but the -1.000 has me stumped. 
Also, my Corsair does not display the minus sign when I remove the VFO
jumper.  But it does display the 1.000.

Assuming for 80 meters the osc/mixer board combines the 7.5 Mhz crystal
and the 5.0 Mhz PTO to obtain 12.5 Mhz, what does the counter circuit do
to eventually provide the display of  3.500.0 Mhz  ?  The -1.000 just
does not seem to play into this calculation 'directly'.

Any insight would be greatly appreciated.