Bakelite Cap on 20x110 Hispano

How common is this black bakelite fuze cap on an Australian 20x110 Hispano?
I have only seen two and both were on empty Dummy cartridges.
Headstamped MF 54 20MM both with factory crimps. One has a primer but no powder.

One has MO on the projectile body (Maribyrnong,Vic).
Is it to keep the cavity clean or to be used after filling.
Was this cap also used by UK factories?

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Haven’t seen or heard of that before.
It could be a production process before filling, though the projectile if HE would usually be painted buff/red.
Did Australia produce any SAPI, SAPI/R rounds?, the SAPI/R shell body was painted black.

Australia only loaded Ball and maybe HEI as I have empty projectiles for HEI? but have only seen Ball loads. I have Dummy loads with unpainted bodies and a fuze plug Rep dummy fuze as a charge assessment cartridge. The projectile shown may have been repainted black? I forgot what I did 25 years ago when I found the first one.I think the second one was also black as shown.
I just wish to know if others have seen this bakelite plug or if it specific to Australia?