What is an "M motor"? (used in a full-scale model of a Corporal missile)

The NASA JPL video Corporal: America’s First Guided Ballistic Missile shows that it used an Acid/Analine engine.

That would have been both challenging and unpleasant to reproduce. So instead, the video below Re-Launching the First Science Rocket to Space says after 02:10 that an "M motor" was used.

Question: What is an "M motor?"

It looks like Estes Model rocket engines only go up to G: https://en.wikipedia.org/wiki/Model_rocket#Performance (from this answer)

A NASA scientist, physics professor, and model rocket hobbyist recreates an historic rocket launched by aeronautics pioneer Frank Malina. The first WAC Corporal dummy round was launched on September 16, 1945 from White Sands Missile Range near Las Cruces, New Mexico. After a White Sands V-2 rocket had reached 69 miles on May 10, a White Sands WAC Corporal reached 80 km (49 mi) on May 22, 1946 — the first U.S.-designed rocket to reach the edge of space (under the U.S. definition of space at the time).

Screen shot from the YouTube video Re-Launching the First Science Rocket to Space

• See the wiki for model rocket motor classification. It goes to class S and extrapolates beyond that. M is 5k to 10k Ns.
– amI
Nov 27, 2018 at 9:55
• I am computer crippled so someone else can answer. Just google 'model rocket motor classification - wikipedia'. M requires level 3 certification by NAR.
– amI
Nov 27, 2018 at 10:04
• I had no idea Estes now went that high! In the late 90s when I was a 'born again rocketeer' with my son, we had to get F and G engines from other companies. Nov 27, 2018 at 13:46
• Just because Estes only makes engines through G doesn't mean bigger ones don't exist. They're restricted products, though, so Estes doesn't bother. Nov 28, 2018 at 2:39
• Bumper 5, a combination of a V-2 as first stage and a WAC Corporal as second stage reached a height of 393 km (244 mi) at February 24, 1949. The first liquid fueled two stage rocket launched in the USA.
– Uwe
Jun 7, 2019 at 11:21