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Does anybody know what kind of operating system is running on the devices?

It's likely a windows-based platform, as Windows has been almost exclusively the platform targeted by memory parser POS malware.

Could possibly be linux (CentOS). I know that at least Autozone's POS system is built on top of that from helping install a few of them.

Target is yes as far as I know a windows shop. All the staff computers are Win7, all the PDAs and wall mounted price scanners are WinCE.