Transmission code question
 

I've been commuting heavily lately so I bought a '93 civic cx with tranny issues ($500). Everything else seems fine except for having it stolen on day 2... but not to get off track of my original question!

The tranny in the car is marked S20 18B P20. I've been googling my brains out trying to figure out if this guy selling his S20 C5B P20, who is claiming it came out of a CX, is full of it or not. I can't figure out what the codes really are meaning but if I had to guess that middle number is the engine the transmission mates to?