Green’s Dictionary of Slang

benny mason n.

also Mr Mason
[such marijuana was kept sealed in Mason jars]

(US campus) particularly strong marijuana.

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[US]Eble Campus Sl. Apr.
[US]Eble Sl. and Sociability 64: Jar pot was one way of referring to ‘particularly strong marijuana’, because it had to be stored in a jar to contain the odor. Jar pot then inspired, by association with mason jar, the synonyms Benny Mason and Mr. Mason.