appliance trap kitchen sink

So I need to replace one of these: These are the replacements I'm looking at: https://www.screwfix.com/p/mcalpine-appliance-trap-white-40mm/81714 https://www.screwfix.com/p/mcalpine-wm15-appliance-trap-white-40mm/5849v My questions are: What is the difference between the two replacement...