If there are multiple printer drivers installed on a machine for the SRP-350 and all + and +II printers then this issue can happen.
When the printer was plugged into the machine it was a SRP-350 (regular) but detected as an SRP-350+. This caused the printer to load the wrong driver and print garbage.
If this happens again (loaner’s would be the most likely for this to happen on) all you have to do is uninstall the printer drivers for the wrong printer and install the Windows 7 printer drivers from Bixolon for the SRP-350. I tested this inhouse and duplicated the issue and fixed it using this method.