Thank you very much for those infographics !
For better or for worse, RA used some main model numbers in the 5370 generation that were similar to old unused numbers in the "Original" line.
That includes the 1769-L30ER and its motion and safety variants. That's a member what I think of as the 5370 "L3" family.
But it can be confusing because those are a hardware generation newer (and use Studio 5000 Logix Designer only) than similarly named controllers, like the 1769-L31, 1769-L32E, and 1769-L35E. Those went up to RSLogix 5000 v20.
I *wanted* a 1769-L10E that fit in the hardware bases for the MicroLogix 1500 and had Serial and Ethernet, supported radiomodem protocols, and had DeviceNet passthrough. I have a treasured PowerPoint deck saying it was in the development process. But it was not to be.
OP's 1769-L31 controller has two serial ports and uses some variation of RSLogix 5000, probably between v16 and v20. One or both of the ports are almost certain to be set for DF1 Full Duplex, so connecting with a serial port and RSLinx Classic will allow the firmware version to be read in the Device Properties, so they can download and install the appropriate version of RSLogix 5000.