I found most of the data from a Blog describing the XM800T's development...
The M-139 was a licensed copy of the French HS.820 20mm autocannon, used in aircraft and AAA. When Oerlikon bought Hispano-Suiza, they replaced their current 20mm design with it. The US used it only for a/c, where it was equipped with 2x 75-rd magazines. (This weapon, while excellent, saw little US use, as we developed the rotary M61 cannon and put that on aircraft.)
Anyway, see the attached...
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