The OP was specifically asking about RT11 on a Heathkit H11, not "just any ol' LSI-11".
Very possible I'm completely incorrect here, but I thought I recalled that DEC gave heathkit a special price on H11 boards and OS license and such - so that Heathkit could produce the system for a price that the home user could actually afford back in the day. And because DEC was highly concerned that someone would buy a card or item from Heathkit and use it in a "real DEC PDP11" (business channel) where it would cost a lot more money normally - they did *something* to introduce an incompatibility of some sort between an H11 and a "real" pdp11? J
