The Lincoln Continental Mark III was designed to be the ultimate personal-luxury coupe, which in turn inspired one wealthy customer to acquire a four-door sedan version.

The story of the lone Mark III four-door begins with Grover Martin Hermann, a man who could afford not to accept no for an answer. The co-founder and retired chairman of the Martin-Marrietta Corportation, the aerospace-and-chemicals conglomerate, he admired Lincoln’s new personal-luxury coupe and contacted the Lincoln-Mercury division about acquiring a sedan model with four doors.

Lincoln had no plans to produce a Mark III sedan, the company wrote back, but it could connect Mr. Hermann with Lehmann-Peterson of Chicago, Lincoln’s official limousine contractor and builder of the familiar Lincoln presidential vehicles. Lehmann-Peterson quoted a price of $13,000—not including another $8,500 or so for a fully-optioned 1970 Mark III coupe donor car—and the deal was struck.

In the L-P shops, the donor car’s perimeter frame, floor pan and body shell, and driveshaft were lengthened 7.3 inches, all between the front and rear seats, stretching the wheelbase from 117.2 inches to 124.5 inches. So while the sedan is large for a Mark III, it’s surprisingly compact for a stretch limousine. Lehman-Peterson also constructed a proper rear seat and appropriate trimmings for the passenger compartment.

In performing the conversion, the L-P fabricators caught a lucky break. While the ’69-’71 Mark III was never produced as a four-door, it shared its platform and body shell with the concurrent Ford Thunderbird, which was. (More on the ’67-’71 more-door Thunderbirds here.) By all appearances, L-P borrowed the complete B-pillar from rocker panel to roof rail from the T-Bird four-door, and the hinges and hardware for the front-opening (“suicide”) doors as well. The Lehman-Peterson Mark III sedan thus has the finished look of a production job, and in a sense it is.

Not long after taking delivery, Mr. Hermann had the Mark III sedan stripped down again by a local California shop and repainted from its original metallic gray to medium blue, the color shown here. According to some sources, Lehmann-Peterson built another  Mark III four-door for Henry Ford II before this one, but more complete reports indicate that most likely, that never happened.

Hermann traded in the Mark III sedan in 1977 and passed away in 1979 at the age of 88. The unique Mark III passed into the hands of private collectors, and when we took these photos at the 2013 EyesOn Design show, it was owned by Phil G.D. Schaefer, an Indianapolis Lincoln enthusiast, and he still has it, we hear. Eventually, Lincoln-Mercury would offer a production four-door in the Mark series: the Continental Mark VI of 1980-83.