You already found port 52773

IRIS does not have http://localhost:52773/RPCNAME

Where did you get it RPCNAME? How did you configure it in IRIS? What else would you expect it to answer?

I tried to find anything about VistA RPC Broker, and found only an installation guide from 1997, It's a year when Caché was released. 

So, you are asking about VistA RPC Broker on IRIS, I don't know how it's possible 

And why it's an issue with IRIS, not with VistA RPC Broker