Dynamic number of fields in a meta box

$fl is ‘quantity’, but from your code snippet it is not clear what $id is set to. I think it is something like 481 or 708. So name="'.$fl.'-'.$id. '" will result in quantity-481 or quantity-708 and not quantity-708-481.

Check the HTML of your metabox if the name is what you expect (use something like FireBug in FireFox or the Developer Tools in Chrome).