The 688 is a compressor, might have been designed for braoadcast use.
Fairchild made it's name from the 670, 660 and 666 limiters, which are highly sought after when in good working condition (and expensive).
They also known for making some gear that was not so great.
Who knows about the 688. If you can find one that is in good working order, it might be worth checking out if you can demo it and get it for cheap.
There's one for sale right now in Encino, CA for $1000 on craiglist.http://losangeles.craigslist.org/sfv/msg/1275865883.html
Since there is virtually no info on these units on the net, it would be hard to say if it's any good.