One aircraft is missing from Apophenia's Marquart list: the MRP-6, built in 1968 and registered in Westminster, Calif. to a Thomas W. Rose (presumably the "R" in MRP-). Its registration was [N1126R] (c/n MRP-6-001) and it was apparently cancelled in 1977. No information about the aircraft, except it used a Lycoming O-235 engine.
I'm still looking for photos of both the MRP-6 and the MA-4 Lancer, but so far I've been unsuccessful. After the prototype MA-4 was built, Ed Marquart worked on fuselages for other people. “The prints are out there for this plane, but I didn’t push their sale,” he commented. Only four are said to have been built, but only one [N4521] (c/n 10) can be found on registers.