Welll at least Jan 1959, which is when Estes started manufacturing the Rock-A-Chutes for MMI.

EDIT
There an MMi ad iin 12/59 issue of American Modeler mag where they advertise 3 standard A4 motors for $0.98....

For this ad to appear in the 12/59 issue, it had to be available perhaps as much as 3-4 months earlier than the publication date

The last MMi ad where they are 3 for $1.50 appears in the 4/59 issue of AM, so that dates them to after 4/59 and before approx 10/59

These were most likely made in Vern's backyard in Denver Co. by Estes Enterprises

Terry Dean

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"Old Rocketeer's don't die; they just go OOP".....unless you 3D print them.