The Masterlock #-series locks are sheer pin locks. That is, in order to set the pins, you literally insert the key you want it keyed to, and sheer the pins off to the lengths that match the cuts in the key.
Basically, this is identical to all pin tumbler locks....
Well, it's immaterial whether the manufacturer chose to use different size pins as replaceable parts, or whether they chose to cut the key pins to the same length when keying. It would be generally better if they would thread their pin chambers, use ASSA-style high-security pins, get the slop out of their pin chambers (tighter tolerance), and tighten up keyways closer to European lock tolerances, so attackers cannot get a rake or paperclip in there.
I understand the typical Masterlocks are considered easy to pick, because they have really really sloppy manufacturing tolerances, also their locks have only 4-pins, where you'd really prefer to see at least 6-pins, they don't consistently use High-Low High-Low key patterns to deter pick access, they generally provide too much extra space in their keyways for ne'erdowells to easily access with a pick.
Anyways, if I wanted a secure padlock.... I would consider ones such as Trioving 5652R, Abloy PL342, Abus disc detainer models, or at least an American Locks lock (After modd'ing to disable the bypass punch).
Better install with the lock a protective cover over any hasp (if applicable)
Don't bother with a >$50 lock; if it's just securing a weak chain, or if the actual item costs not much more than the lock.