@ScruffR I've also tried this on a windows machine. Similar to linux older deviceOS doesn not seem to appear when I connect or when I do a scan. The scan is done using the "mode" command on CMD or PowerShell. Any suggestions to detect the COM/ACM ports for older device OSes?
I was also referred to manufacturing scripts by particle GitHub - particle-iot/solutions-manufacturing . Quick look shows that they too use the 14400 baudrate trick to trigger DFU mode. However, it seems they used stty commands to force the baudrate. This looks a bit weird to me since 14400 isnt a redefined/common baud rate and from the post How to change baud rate to 14400 on Linux? this shouldn't work. Even if the stty command works, I think the issue of detecting which ACM port still exists.
I'll probably look into this script in detail this week to see if they do anything special.
I also found this post Factory flashing using Workbench by @rickkas7 which describes excatly what I want to achieve. Here he mentions the use of SWD/JTAG which I guess I could try if I can't figure the USB issue out.