Seeing it on your PC
System (PID 4) listens on 139 on adapters with NetBIOS enabled.
Should you block it?
Never expose it to the internet. Modern Windows file sharing works over 445 alone.
Check it on your PC
Run this in PowerShell to see what is using port 139 right now, and which program owns it:
Get-NetTCPConnection -LocalPort 139 -ErrorAction SilentlyContinue | Select-Object LocalAddress,LocalPort,RemoteAddress,RemotePort,State,@{n='Process';e={(Get-Process -Id $_.OwningProcess -ErrorAction SilentlyContinue).ProcessName}}No output means nothing is using the port at the moment. For every connection at once, paste netstat -ano into the netstat explainer.
Block it in Windows Firewall
In PowerShell run as administrator, this stops other computers from connecting to this port on your PC. It does not stop your own programs connecting out.
New-NetFirewallRule -DisplayName "Block inbound TCP 139" -Direction Inbound -Protocol TCP -LocalPort 139 -Action Block
Undo it with Remove-NetFirewallRule -DisplayName "Block inbound …", using the same name. If you depend on the service on your local network, this rule stops that too.