While your image is poor for the purpose of evaluating your coin, the scale does seem to show a light coin. The question is: How did this happen?
Here are a few possibilities, which when combined, could easily shave 20% off the nominal weight of your 1973 Lincoln.
1: Your coin appears to be very worn with circulation.
2: Your coin appears to be pitted from lengthy exposure to an adverse environment.
These observations, coupled with the very real possibility of the coin leaving the mint a bit light (5% to 10% light was not uncommon for .950 copper cents) can easily add up to a total of 15% to 20% light. The effect of wear can be extreme. I have Lincoln from the teens and twenties that weigh in just over 2 grams, and a few wheats so worn that they are barely recognized as coins.
Personally, I do not think you have anything special. Were it not so worn, and was over 20% light, I could be persuaded it change my mind.
Time for more coffee.
