I have an old and reliable USR Courier External (US/Canada version; 20.16Mhz.) that I had upgraded until v.34+, but -stupid of me!- I missed the "official" upgrade to x2 and then, to v.90.
Last year I have tried EVERYTHING to obtain/purchase the upgrade from 3Com/USR, without any success (the worst thing that could happen to USR was to be bought by 3Com). The 3Com/USR damm** people have "pinballed" me several times from their US sites to the European sites (my modem is US/Canada, but I live in Spain).

I was to give up all when I find this site --> http://www.freelsd.net/

There are an patched x2 firmware that NOT NEEDS KEY to enable x2!!!!
I have tried and it have worked OK in my modem (when I tried it I have crossed my fingers, thinking that I was to screw the modem. It doesn't, but I don't know if it can also work in all cases. So, please, don't blame me if you try it and something goes wrong).

This firmware have some "pluses", according to their autor: several bugs corrected (I don't know if this is true) and the ability to change your modem zone (this is real, I have changed mine from "US/Canada" to "International". After that I'm been able to upgrade the modem to v.90 with the International firmware that I got from the spanish 3Com site!!!)

If you have an v.34+ Courier @20.16Mhz. (NOTE: the firmware is NOT for 25Mhz. Couriers ), this site deserves a look.

NOTE for edu1: Originally, my modem was "US/Canada" type. You don't say what thipe is yours (I don't know if the firmware in this site can be uploaded to an different type). Maybe anyone have some idea about...