The string is a time capsule. It tells us:
The firmware string 3.16.0 0.9.1 v6031.0 Build 210407 Rel.7370n -TL-WR850N- represents the final, mature software state of a budget router released in the mid-2010s. 3.16.0 0.9.1 v6031.0 Build 210407 Rel.7370n -TL-WR850N-
The "v6xxx" series of versions (like v6031.0) usually identifies firmware tailored for specific operators using TP-Link's Agile Config. Maintenance: The string is a time capsule
If you see a string like 3.16.0 0.9.1 v6032.0 Build 220101 Rel.7380n , it is a (2022) but for a possibly different hardware revision. Do not cross-flash. 3.16.0 0.9.1 v6031.0 Build 210407 Rel.7370n -TL-WR850N-
Unlike the blue/green UI of older TP-Link routers, this build likely contains the . It is touch-friendly but lacks advanced QoS settings.