Well I studied the powerbook duo modem connector, its a datapump connector hooked to the bus, and not a serial port, at least not that I had seen. In order to put a serial port thats usable on the modem connector, you would nee one of apples VIA chips i guess.

And that converter goes 1 direction, it gives USB computers a full RS232 serial port thats TTL compatible. For Arduinos and other crap. You could use one to plug a regular mac into your PC USB port for doing null modem file transfers, or if running XP, Basilisk localtalk access.