I think the issue is that there is a higher probability of getting an older one ( 1021 serial number model) being in tune than a newer one. You also have to take into account how many 1021 models were made versus the new ones. If there are more newer models, then it stands to reason there is a higher probability of one of them being out of tune.
Best way to check is the air test on a clad dime. With sensitivity set at 4.5 or a tad higher ( stock 8 inch coil), if it can hit the dime with a repeatable high tone at 9-10 inches, then you have a good one. Then again, there are guys hunting with the newer ones where it does not air test as well but in actual in ground tests, it rocks....so ya never know.
JC