Severely underweight Antoninus Pius

This coin was part of a lot of 10 coins. It weighs 4.74 grams, and is 27mm across.
Ant ideas why it is so light. My first thought was a fake because it is also very thin

 

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A sestertius of Antoninus Pius and Indulgentia has a similar design. Yours is not a sestertius, but perhaps a corroded As, which would explain the low weight.
Here is the description of the sestertius:
http://numismatics.org/ocre/id/ric.3.ant.891

 

All ten coins in the lot were described as AS, sorry, forgot to mention that.

 

I'm assuming that's an AE As. (ANNONA AVG?) The surfaces are eroded and it has a lot of edge loss. Could be that it's also lost mass as some of the alloy has leeched/dissolved/crystallized a bit.

So, if it started out at maybe 8 grams -- a somewhat lightweight but not extraordinary example -- it wouldn't be that hard to lose 40% of its mass.