The Ford Fairmont and Mercury Zephyr can easily be mistaken for an "M" body to the untrained eye.  Although slightly shorter (The also-small Ford Mustang was based on the Fairmont the same way the Diplomat et al was based on the Aspen/Volare) the visual resemblance is undeniable when other Ford models of the era are compared.  The Fairmont is a long-running Ford nameplate, and Fairmonts are still available in Europe and Australia, resembling a sleeker version of the Crown Victoria.  The genre of Fairmonts herein were delivered to showrooms at the same time as the the Diplomat beginning a year after the Diplomat debut in 1977, but was cancelled after 1983.

Ford Fairmont/Mercury Zephyr

If there are other imitators or imposters you are aware of, let me know.