The only thing that comes to mind is that those might be the original programming port. Until a few years ago, (around the time of the DST date changes), one needed to purchase or rent an updater from Davis which installed on the Expansion connector and connected to a PC to upload new firmware. Then the firmware itself was changed so that future updates could be done through the serial interface (probably USB, too).
(I rented one from Davis and then sub-rented it to a few people on one of the regional weather networks before I returned it.)
Hmm, I suppose that's something I should test on my serial interface. I wonder if it'll brick the console...
Yup. This is the only thing I've ever seen it spontaneously burp out. If you can figure out what the other serial port brought out to pins 11 and 12 of the expansion connector do though, you'll be ahead of me. Maybe C2520 knows?