thinkpad 390x, how to change backup battery?

[oldmanstool]

hi,
i do use an IBM thinkpad 390X (2626-H0G) that loses its settings of time after each shutting down.
i expect it to be caused by an empty backup battery.
i have found a pdf file (ThinkPad 390X / i 1700 HMM) (http://download.lenovo.com/ibmdl/pub/pc ... 9n1083.pdf) with explanations and drawings how to do FRU procedures.
it has pics how to remove parts of these machines.
it shows a lid on the back of the thinkpad to access the internal modem card, from my understanding of the pdf, the modem card should be removed (as well as the main batterypack) to get to the backup battery (RTC).
my machine has a modem inside (is detected by my DSLinux OS as a Lucent Winmodem), and there is a connector on the side of the thinkpad marked with a telephone icon.
so it looks as if the modem is present.
PROBLEM: the lid that seems to cover the internal modem card is screwed off the body of the laptop (looking at the doc above) by taking out 1 screw, but on MY machine the lid is present, but NO SCREWS are...
on one side of the lid, not on the side the screw is pictured in the pdf, but on the opposite side, there is a small round recess in the bottom cover of the laptop, a bit bigger as the diameter of a screw head (comparing it to screws i can see on the back cover).
it COULD be a 'hidden' screw covered by a plastic or rubber cap(ping).
i do not want to force (and destroy) anything on this lovely machine, so i am unsure how to go on.

any advise on this from the GURUS?
or did anyone the same task on an identical machine?

thank you

[RealBlackStuff]

1) remove the main battery.
2) remove the modem lid
3) remove the modem card
4) replace the CMOS battery (CR1220 I think)

As to 2), in the middle along the long side near the rear of the computer, should be a screwhole, which may be covered with tape or a piece of rubber.
This is what the lid is supposed to look like: http://www.ebay.com/itm/370737812466
390, 390E and 390X all use the same lid.
And here you can see where the battery is: http://www.ebay.com/itm/160773798164
(round silver 'coin' on the left)
But maybe a previous owner replaced it with a different lid, and you may just need to gently 'pop it up' with a small screwdriver.
The hole you mention is most likely for removing a keyboard screw, definitely NOT for modem/RTC.