Seems 2B3 is the most sold one. Latest FW available is V1.2, built 2004 08 10 xxxx (as you wrote above), available at November 2004 - no futther update.
Did many tries to use 2 modules on 2B3 - indentical, mixed manufacturer, mixed size - nothing was sustainable stable. Sometimes 2x1GB modules seems to work, but after a while I had strange behaviour on different programms (eg. OWB, Blender, etc.) and complete crashes. Since years I now only use a 1x1GB-module, which runs very stable.
Afaik only rev. 2B5 is capable to use more than 1GB, but it's very picky and it must be 2 identical modules.
Rev. 2B1 I've never seen and it's new for me, that this rev. works with more than 1GB too......