It seems like the current emulation has 2 2SIO boards and the telnet port maps to them in sequence. The user id seems to come from the emulated physical port the user connects to. Being S-100, nothing really prohibits having 8 2SIO boards for up to 16 connections (provided their emulation allows us to set different IO addresses). It looks like the Interfacer 3 and 4 used a register to assign more than one user to a single UART IO address, allowing (on the 4) up to 32 terminals on 4 UART addresses.