Simple. Mags have to have a SN and date. If they don't, they are illegal. It will only take them about 5 minutes to figure that out and write it into the law.
Not that simple. There is no way of going back and dating all magazines that have been made and owned already, so saying that all without a date are illegal will not work. What happens when one is found in your possession that was purchased last year with no date or SN?
They could write in that all mags sold in CO from this date forward shall have a date and sn, but good luck getting manufacturers not in CO to comply accross the board on all magazines. They could simple date the mags less than 10 rounds for CO and not date the rest for other states. Then good luck proving that the mag was not already owned.