As far as when the radio was made look on the back of the radio at the serial number, the first three numbers are just a model number, the Z indicates that out of all of the Uniden products it is a scanner. The number after the Z, the fifth number, is the date of manufacture, 9 would be 2019, 0 2020, 1 is 2021... the sixth number is where it was manufactured and that will always be 8 for Vietnam unless it is a beta engineering unit and that would be a 2. Then you have zeros and numbers this is the number sequence of manufacture and this number starts over every year.
As far as the Hum goes, I do not know if it was ever addressed on the production line but I have asked the Uniden product manager if they could take the most recent serial number Radio, take the cover off and look... perhaps a member could do it where the Hum has not showed up after 3 or 4 months of purchase and get back to us.
One thing for sure if the repair has been done properly and permanently they will no longer need the mitigation device.