Sorry for resurrecting an old post, but I know two more of the RAM expansion exist, as I own two of them. So, 3 at least.

38kB is available because: $0000-$07ff is zpage, stack, screen RAM, temp buffers, etc. (2048) $a000-$bfff is BASIC (8kB) $c000-$cfff is RAM, but can't be used by BASIC (4kB) $d000-$dfff is IO (4kB) $e000-$ffff is KERNAL (8kB) So, 26kB is lost to those things. That leaves 38kB available for BASIC. BA...

It's somewhat of a gamble. The 264 is rare per se, but it's not all that much different than a production Plus/4 (unless you have found an early prototype, which might have more visible differences). Thus, it's of most interest to a collector. I know Bo Zimmerman is looking for one, so you might try...