It would probably be half a megabyte of RAM with no real-time clock.
I see from this
http://en.wikipedia.org/wiki/SIMM wiki that 30 pin simms were available in sizes as small as 256kb, so your guess could be correct about the size of the RAM expansion. I did not know that 30 pin simms were available with that small amount of RAM installed on them. My smallest ones are 1mb each.
