Weight and thickness will tell you, should be about .06" or 1.5mm thick and either 2.5 or 3.1 grams, odd how the Memorial is visible on obverse.
this one sure look like it may be a real mint error. the planchet may have split apart after the strike.
Isn't this a pre-82? I thought it was 1 solid piece, how would it split? I am not questioning you, just trying to understand how that would be possible.
Planchets that split after striking normally occur on solid metal alloy planchets, like the pre '82 bronze. Take a look at this link, it should help. http://error-ref.com/Split_plan_after.html
this one looks like a full brockage should look but since the op said it was under weight and thin I thought the planchet may have been defective and came apart. I'm almost certain it is a real mint error.
It looks like It is struck through a mid to late stage die cap. Faint traces of the lincoln are showing through an enlarged distorted memorial. That the coin happens tobe underweight as well is just a bonus. It is not struck on a split planchet. If it was and the planchet split before strike the rev would be much weaker. If it split after strike the rev would be strong but the obv side would be much rougher, not that smooth.
They're not as rare as you'd think. They pop on on eBay pretty often. They're much more common than full-thickness, in-collar uniface strikes.
