Re: MMSI into Icom IC-M504
Don't overthink this. It is what is is. Either follow the law or don't.
BTW, in my judgment -- over 50 years of sailing on the Chesapeake, 100-ton license, etc., etc. -- there is absolutely no reason to call a tanker, anyway.
Stay the hell out of their way. You can see them even if they can't see you. And, you have plenty of time to make necessary course changes to stay clear.