There was much done before the focus on the M-20 and later. The M1 Eaglerock was sort of like the DeHavilland Tiger moth. It wasn't until Mooney joined with Culver to really get going on marketable and well performing aircraft. The M-6 was low wing monoplane, it never got much past the design and testing stage.
After a stint with Bellanca, Al moved on to some success with his design about M-12 or 13 called the Dart. Variations of the Dart sold about 80 planes. Very few remain. Then, joining with Culver Al brought is clean aero design to the M-14, and along with Culver a list of several M series begat the successful Culver Cadet. Cadet having a manually retracting gear, and clean lines with elliptical wings, was ready at the start of the war. More than 600 Cadet variations were built for civilian sales. The Cadet came in 75HP Lycoming moniker LCA, and Franklin engines called the LFA.
A mildly modified Cadet was offered to the war dept as a target drone, and re-designated the PQ-8. This would possibly be about M-16, although the structure and basic elements conformed to the early Culver Cadet, and it took form from back in the Dart G progeny. The PQ-8 target drone and the TDC-1 and 2 for the navy were built in huge numbers(for the time). Accurate production is hard to pin down, but estimates are about 1400 of the PQ-8/TDC's and about 600 of the next version PQ-14. Variations of the PQ-8 entered civilian production post-war but were not commercially viable. The PQ-14 could be considered the Mooney 'M-17'. Noting that the M-12, back in the Dart up to M-17 were very similar in design and construction with just mild evolutionary changes and no major structural upgrades.
Fixed tricycle gear allowed the drones to take off and land easier, larger engines, props, all moved up to the next well known M-18 Mite. Single engine, single seat, manually retract. 283 built over 7 years. At the end of the M-18C version, best guess is the M-19 was the first iteration of a four seat plane. No production is known, and it may have only been a mock-up, or 'buck' for the fuselage and engine placement. And, on to the M-20 versions up through the Ovation.
Al had a lot of design experience that shaped the M-20 and later. Sadly, not a lot is known of the early designs.