alt Hacker NewsHe doesn't mention it's a RS485 I/O card meant to fit into an CEC expansion box in a Z14 mainframe. Might be helpful terms to search if you're looking to find very similar parts...since there might be run on this one specifically. Found part number 98Y6848 looking this way, which seems like an updated (or maybe just renumbered) version of this.
RS-485 is pretty much the standard to communicate with SDLC to another device. Think of this board as a serial port for a mainframe. Looks like the same card is intended for use in IBM POWER hardware too, running OS/400 or AIX (or Linux).
Of course, an open question is who on earth is still using SDLC over RS-485 these days, but then again I still see new Dell servers fitted with RS-232 ports.