I can't seem to find it anywhere on them interwebs.

I don't believe there ever was a 3.90.6. Perhaps you mean 3.90.3 or 3.96?

IIRC Lame 3.90 development was a little confusing in the end, but I think 3.90.3 was the last 3.90 version.
IIRC there are no significant differences between the 3.90 versions, and even 3.91 and 3.92 (maybe also 3.93) were very similar to 3.90 (not a surprise because IIRC 3.90.3 came out nearly at the same time as 3.92). Significant changes started after 3.92 (or maybe 3.93).

after 3.93 from what I remember. Ok yeah, you're right pdq. 3.90.3 was the last from that, and if i remember correctly is based off 3.93 (but i'm probably wrong, lol)

fyi, i found them here on John33's server
http://homepage.ntlworld.com/jfe1205/LAME/lame-3.90.3.zip
http://homepage.ntlworld.com/jfe1205/LAME/...e3.90.3_MOD.zip
http://homepage.ntlworld.com/jfe1205/LAME/...l3.90.3_MOD.zip

The oldest lame source I have is 2.3!! And I have pretty much everything, in source form, from then on. So, if you want anything a little unusual, I probably have it!!