I never saw two implementations that were the same either, it was a particular annoyance, but they all defined the PAN as n..19 IME (and 19 was indeed a rare ca

I never saw two implementations that were the same either, it was a particular annoyance, but they all defined the PAN as n..19 IME (and 19 was indeed a rare case generally seen with now defunct/less popular card schemes so there's proabably still room to fill). It's true that the number of organisations that interact with the message formats for sending card transactions is significantly smaller than other things, but we're talking about every single bank, every single PSP, every single embedded card device, and probably many more organisations beyond that, currently in operation today. Given the glacial pace of most banks' IT operations I would not expect them to do be able to achieve this within a time frame of several years or perhaps even a decade. Sure these are not the same challenges as IPv4 and such, but it is in no way a trivial change to tweak a message format that has been in use since before most HN users were born :)