Questions: HP 8640B signal generator . . .

Greetings all,
I picked up a used HP 8640B signal generator off of eBay. I have always
wanted one, and now I have one.
Mine seems to work okay, but the tuning cavity only requires use of the
clockwise most portion of its range. Much of the counter-clockwise range is
just not needed - even though the frequencies seem to be okay. Is this
normal? I have never used an HP 8640B before so I cannot say.
Also, I noticed that when I remove the top-most counter board (the board
that contains the digital readout segment displays), I see a smaller
compartment that contains the pre-scalar. Judging by the missing screws, and
the overall appearance, it looks like there could have been a lid on that
compartment. Was there in fact such a lid? Or, is this locations reserved
for an option? Also, after removing the top lid from the unit, I see two
strong "RF boxes" the smaller one, in the front, is the frequency counter
(with the 7-segment readout as described above), and the other box, the big
one in the middle right-most side (looking form the front), is presumably
the frequency multipliers. Now my question is, behind these two boxes, is
another (much smaller) open-lid box that contains a few plug-in cards. But
when looking closely at this box, I see that it too might have had a lid.
Can anyone say for sure if such lids were in fact present when the unit was
new? If so, can anyone provide such spare parts?
Oh, and while I'm at it, can anyone say if it is possible to add option-002
(the frequency doubler option)? Can such parts be obtained - perhaps from a
dead unit? Or am I better off just finding a unit that has the doubler
option already installed and parting with my current unit?
Thanks,
- Stan Shankman
K7SJB