Registered as "Exasoft IP Port"; nothing further about it is public.
IANA registers 3920 as exasoftport1 to a private assignee. There is no published specification, vendor documentation, or open implementation for it, and it does not carry a well-known malware or scanner association. The registration is the extent of what can be verified. Trace any listener on 3920 back to its process rather than reading meaning into the port number.
Expose with care
Nothing is published about the protocol, so there is no basis for trusting it on a routable interface. Identify the owning process and keep it inside the network until you know what it speaks.
$ sudo lsof -nP -iTCP:3920 -sTCP:LISTENSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.