I still can't think why setting the default Symbol Set to the Roman-8 set (8U) should make any difference, but the observed symptoms are perhaps explained as follows:
- The second command-line downloads the contents of the 'second PJL file".
- Those contents start with the Universal Exit Language (UEL) escape sequence, which puts the device into PJL mode.
- The printer processes the following PJL commands, but does not recognise (and hence just ignores) the first one (@PJL OKIWILDFSDELETE NAME="0:\fonts\*_1") because it appears to be proprietary to OKI devices.
- The second PJL command (@PJL FSDOWNLOAD FORMAT:BINARY SIZE=18280 NAME="0:\fonts\OKI-CODE39-regular_1") is also almost certainly ignored, because the LaserJet M401 device does not have any suitable permanent storage (e.g. hard disk, DIMM) available.
- That command (should be) followed by 18280 bytes of binary data, which will be in the form of a PCL5 soft font download.
- That download will (assuming it is in a recognised format) store the font as a temporary resource in the printer's Random Access Memory; the font should be shown in a PCL5 Typeface (Font) List if one is invoked immediately after downloading the file.
- The temporary soft font will be available for use (which might explain why it works once), but will then be deleted from RAM when the next UEL is received (usually at the end of a job, or the start of the next one), or when the printer is powered-off (or, I think with some current models, once it enters sleep mode), or if a print job contains PCL escape sequences (or PJL commands) which cause soft fonts to be explicitly deleted.