Re: MMSI into Icom IC-M504
In the USA, Boat US took on the issuance of MMSI numbers for recreational boats. They did it as here in the USA, you don't have to have a radio license issued by the FCC to operate a VHF as long as you are staying in USA waters or you are not communicating with foreign flagged vessels. Hence the two numbering entities.
When I switched MMSI numbers, I was able to change the MMSI number on the radio (second entry) and canceled my Boat US number on line. It's pretty easy to do.