Those with standard ports are listening nodes which will show up when you add the node, otherwise adding the node makes no difference to the getpeerinfo result (that's also how you can tell). This is because if a node is configured to listen on a port different from the standard 41682, you have to add the ip address and the port in order for your client to treat it as a potential listening node (and potentially sync from it). Otherwise, only nodes listening on the standard port are checked by the clients for being listening nodes and those that aren't specified as such (via addnode) are assigned an arbitrary port.

F'rinstance, the IP address (144.76.64.49) of my old (should have been decommissioned months ago) Hetzner host is listed in the strDNSSeed array, the list of supposedly-stable peers that a fresh node consults first when syncing from zero. However, I'd turned off the Slimcoin node because I'm working towards decommissioning the host. In the interests of stabilising the network, I reinstalled the Slimcoin node and synced up to date from a few-days-ago chain snapshot "ip" : "144.76.64.49" "blocks" : 2362979 with the (additional to the hardcoded strDNSSeed nodes) addnode=185.150.190.19:41682 and addnode=185.68.67.37:41682.

My client on the new server (144.76.118.44) sees the newly-started client on the old server as "inbound: true" and has assigned it a random port because I haven't told it otherwise (via addnode)

Also note that the blocks field of getpeerinfo is incorrect and unreliable , it reported the block height incorrectly as 2361253 and not 2362979.

I then logged on to the new server and executed addnode 144.76.64.49, then getpeerinfo with the result:
The addnode command caused the node to re-check, changing the port, updating the send/conn times changin the "Inbound" status and updating the "blocks" value to the correct one.

Right now, nodes worth adding are the listening nodes: 185.150.190.19, 185.68.67.37, 144.76.64.49, 144.76.118.44

And, just for comparison with before in terms of chain movement:

Cheers

Graham