Maybe the weight in grams. I seem to recall something about the weight of steel cents changing during production. Not sure though.
The small cent Lincoln except for '43 and the zinc Lincons weigh 3.11 +/- .13 grams tolerance, so from 2.98-3.24 grams. The steel cent is listed at 2.70 +/- 0.13 gram ( Breen reported that there were 2 different weight planchets for the steel cents, but both were within tolerances, so no way to tell). Zincolns weight is 2.5 +/- 0.1 gram ( weights from Breen's). So the 3.2 small cent seems to refer to pre-1982 bronze cents and the 2.9 probably refers to the high end of weight for the 43 steel cents. Usually such can come up when trying to ID a bronze '43 error, or a steel '44 error, along with magnetic vs nonmagnetic.
2.9 grams would be just a little over tolerance for a steel cent. the standards for the steel cent were a little loose with as mentioned two weights 41.5 grains (2.689 grams) or 42.5 grains (2.75 grams) and with a .13 gram tolerance. So the heaver standard at max tolerance would be 2.88 grams. If you weighed that on a scale that only went to one decimal place it would read as 2.9 grams.
I have a 1943 Steel penny. That sticks to the magnet. But it does not weigh 2.5 grams it weighs 3.1 grams the same weight of a copper penny. Can any one tell me is it worth anything.
First, you need to check your scales with the calibration procedure for your type of scales, Second, an image of the coin on the scales with the weight showing is needed. They weren't 2.5g as you say, but between 2.7 and 2.75 with a tolerance of 0.13g.
I went and bought 2 more scales and picked up my friends 1943 penny that weights2.5 grams and mine still says 3.1
I have one 43 steel penny that weighs 3.0 gm and it sticks to a magnet. Could it be a large planchet?
I also have a 1934 P penny where the last T in TRUST in on reverse is not struck. Then there are lines going down the coin across the 3 that the bottom half of the 3 is flattened, not raised as the 1, 9 or 4
Heavy 1943 pennies are probably just plated. The plating just adds additional weight to them, is all.