vhf MMSI number

[Hurricane]

I recently sold a Standard Horizon hand held and needed to clear the MMSI.
Dependent on the version number it’s either a back to factory job or fill in a website form. It took 2 weeks for SH to come back with a code but straightforward after that.
Maybe Simrad can do the same

I think we are talking about the VERY old RS8400DSC unit.
Like this

These units had jumpers inside to set the various modes of operation.
IIFC one of those jumpers cleared the MMSI setting back to factory.
After setting the jumper and powering up, you powered down again.
Removed the jumper and powered up again.
You could then follow the manual for setting up the MMSI
I don't have my full notes but I do still have the two manuals - one for the handset and one for the DSC unit.