None that I know of, and we're using routers built in '92. The only WAN
interface that I know that had a default MTU size of 576 is a slip
interface, which *was* popular back when the internet was just a collection
of Universities and research centers connected by 56k frame relay circuits.
Everything anyone is using out there that's connected at T1 or better or
more recent data lines like ISDN/xDSL/ATM at a minimum has a MTU of 1500
only because that's the max MTU size of Ethernet, the actual links
depending on the protocol used potentialy have much higher max MTU sizes.
For example, frame relay has a max MTU of 1600, PPP links have a max MTU of
18000 (yes three zeros). The reason for 1500 is for 10bT Ethernet which is
the most common LAN medium currently. 100bT Ethernet has a larger max MTU
size, but I don't happen to have a manual near by that has that value in it.