1956 2.22 grams

[GrumpyEd]

I also think it's an acid job.

I've noticed they also have a different sound.
When dumping bags sometimes I hear a funny clink and it's often one of those.

Tell tale signs are thin letters and orange peel surface and slightly reduced diameter.

[mustbebob]

Good dialogue here. I would rule out a split planchet pretty quickly.
My opinion is that it is indeed an acid coin. The acid would reduce the surfaces uniformly meaning the same amount of metal that was eaten away from the field, would have been eaten away from on the devices. You also need to consider what the acids' strength was also. Of course, this is all my opinion, but after seeing so many of these, that's the way I am leaning.