A Windows machine with media streaming enabled — HTTP.sys serving the WMPNSSv4 endpoint.
Not registered with IANA, but on Windows it is entirely predictable: the Windows Media Player Network Sharing Service reserves the URL http://+:10243/WMPNSSv4/ with HTTP.sys under the NT SERVICE\WMPNetworkSvc account, so the port opens whenever media streaming or DLNA sharing is turned on — often by a setup wizard rather than a deliberate choice. It is the HTTP half of the UPnP AV media server, advertised over SSDP on UDP 1900. Nmap sees it open on about 0.07% of scanned hosts, essentially all Windows.
Do not expose
A LAN media-sharing surface with no business on a public IP: it exposes the library to anyone who can reach it and puts HTTP.sys, historically a source of remote vulnerabilities, on the wire. Turn media streaming off if you are not using it.
$ curl -sSI http://TARGET:10243/WMPNSSv4/Seen 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.