Handy guide - thanks a bunch. I have the similar model that I picked up used without the WWAN option and I've been researching what it would take to add one. Near as I have been able to figure out that second slot is in fact the one for either the WWAN or other WiMax and similar type connectors. Presumably Dell also throws on some kind of a sim reader at the end of the slot underneath and probably something for an antenna. But without seeing a teardown of the WWAN enabled model, it's hard to say what is entirely needed to retro-fit one with the other.
Obviously the extra PCI-e is on the other side of the PC board from where the sim is located which seems par for the course as most PCI-e Gobi devices do not have a sim slot on them. I've seen a number of ribbons for sale with the sim-reader on the end, but again, without having one of the wwan pci-e devices in front of me, I'm not sure if the ribbon connects directly to the wwan card or somewhere on the motherboard. This raises the question of compatibility on various pci-e wwan options, as the one dell had with this is 3G which will rapidly become out-moded. There are newer PCI-e WWAN devices available with a similar footprint that might work if you could figure out a way to install a card reader and get an antenna in there somehow.
Chasing down the part numbers of these from Dell was not too difficult although I had to go through half-a-dozen members of the support staff until someone could find them in the system. I suppose I could try it again to see if they can find the SIM reader and anything about the antenna for these devices.
If anyone has any more information please let me know. I had to tear into mine because the original over-heated one of the tabs on the battery door. I sent it in for service and they replaced some of the over-heating components but sent it back without replacing the actual bottom cover which had the melted tab on it (the battery door wouldn't stay shut). I sent it back and again they returned it unfixed.
So I eventually just had them send me the bottom and I swapped the parts out myself rather than go without the laptop for another few weeks and risk having it come back unfixed yet again.