Australian 20mm Hispano case with side stampings

A friend of mine has a fired 20mm Hispano case made by Footscray in 1955 (HS: MF 55 20mm). The case has stampings on the side, close to the head. They are at 90° to the case centre line.

Top row: ME 3 57
Bottom row: BL Iz

Am I correct in guessing the top row indicates that the case was loaded in March 1957 at the explosives and pyrotechnic factory at Maribyrnong?

I’m assuming that BL Iz refers to the propellant but is BL the manufacturer or the propellant type or lot number?

I’ve not seen this on 20mm Hispano cases before but I haven’t seen a lot of them and all of mine are either British or US made. When did this practice commence/cease?

BL will be for BALL.
Other stampings to be found would be HEI, SAPI ®, SAPI.

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Such stamps also appear on UK 30x113B.

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…and 50’s dated UK 20mm Hispano

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I have only seen Australian 20mm Hispano Ball loads with the side stamps from 1949 to 1959.
MF 49 20MM is dated 3 50.
MF 55 20MM is dated 55, 58 and 60 so old cases were loaded whenever needed?
MF 59 20MM is loaded 4 60.
All are BL. I wonder if MF ever loaded HEI or other types during these years?

The earlier MF 1942 20MM , MF 43 20MM and MF 44 20MM were not side stamped.