Unregistered, unnamed, and rarely seen — there is nothing standard behind this number.
51493 is in the Dynamic/Private range, so IANA holds no assignment for it and RFC 6335 says none can exist. Nmap's service database lists it as unknown with a low open-port frequency. No canonical source ties it to a product or protocol, which makes "an application chose this number" the complete and accurate answer; the host's own process listing is the only authority on what is listening.
Expose with care
Treat as unidentified. Map it to a process locally, then apply whatever verdict that service actually warrants.
$ sudo lsof -nP -iTCP:51493 -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.