Nice find!!! Congrats!!!
I think you measurement is in grams, not grains. 15.432 grains per gram.
A typical 69Cal roundball measures 0.64-0.66" diameter and weighs around 23-26 grams, 355-401 grains.
The only 75Cal I have ever dug measures 0.723-0.742" diameter and weighs 36.85 grams, 569 grains.
These numbers above come from my finds log measurements.
If yours is 42.4 grams, 654 grains, that is huge for a 69Cal roundball and doesn't correlate to a 0.69" measurement. Might should double check those measurements.
I remember you posting a 69 a while back. Do you still have it for comparison sake?