Paul,
I've come across the T slot outlets more or less anyplace you could put an outlet in older homes dating from the late 1800's to about the early 1960's.. The house I grew up in was built in 1958, & peculiarly, all the switch controlled outlets were T slot, while the rest were standard Nema 1-15.. I've personally never come across one of the T-slots wired to 240V, even though some of them show a dual 120/240V rating.. I've also come across old 1-15 outlets which show 250V ratings!

Then there's the really odd versions that basically function the way a T-slot would...

[Linked Image from pstr-g02.ygpweb.aol.com]

I've only come across a handful of these..

-Randy