It is a Mpi Miracle Switch. p/n ACC407. I just purchased one at a hobby shop.

It is a 'fail safe' design with a built in 6V regulator that can draw up to 0.05ma when turned off. Mpi recommends that you disconnect the battery if you plan to let your model sit for more than 30 days.

The current version has a fancier case. [link=http://www.maxxprod.com/mpi/mpi-21.html#Miracle%20Switch]Miracle Switch[/link]

Stan