There is no back plate, but the female portion of the plate is held in place by 6 screws and if fair sized stainless washers are used, will stay flat and not leak with a minimum of caulk. This is the only way I have been able to get that last 3/4" of water out of the tank even tried traveling with drain open. Here is a link for one version.
http://www.westmarine.com/webapp/wcs...classNum=12946