Just a little follow up here, in case anyone else runs into this situation -
ACR sells two different versions of these things. They are the Rescueme MOB1 (which may actually be made by Ocean Signal, judging by the manual I downloaded) & the AISlinkMOB, which is their traditional colors & may actually be made by them, or may be a better brand labeling job. Both are roughly the same price. Both use the same type of computer software to enter a User MMSI #. Apparently, other companies make similar devices too. This is a link to a webpage that compares some of them -
https://www.panbo.com/testing-ais-mob-beacons-acr-dsc/
Below is an excerpt from the ACR manual that applies to my original question. I'm still not 100% sold on using the boat's MMSI number in the MOB unit, but that seems to be what they recommend. I am also still not clear on the difference between the User MMSI & the pre-programmed "self identification number" that can't be changed. I did not find a reference that tells me if you can change the user MMSI number multiple times or not. The spec sheets for both the ACR units say that the units put out 1 watt for AIS & 1/2 watt for DSC. This combined with a small antenna that will likely be below wave height, should add up to a very limited range. They warn against testing the unit more than a few times per year to prevent depleting the battery. If the unit is used one time, even for a short time, the battery must be replaces at a factory authorized facility. Shelf life is said to be 7 years.
Excerpt from manual:
4. MMSI configuration
If your vessel has a DSC enabled VHF radiotelephone, it is strongly recommended to
programme your vessels MMSI number into your MOB1. This will allow the MOB1 to send
the details of the man over board incident direct to the radio and sound the alarm.
4.1 Self Identification
The MOB1 is supplied with the self identification number pre programmed. This number is
specific to each MOB1 and cannot be changed. The MMSI displayed on the DSC receiver
will always start with ‘972’ irrespective of the country it was purchased in.
4.2 User MMSI
(Applies to DSC enabled units only)
To be able to send DSC messages to your vessel, the MMSI number of the vessel needs to
be programmed into the MOB1. This is achieved using a PC based application and its
display screen.
4.2.1 Installation
The programming software for configuring the User MMSI into the MOB1 can be
downloaded from
www.oceansignal.com/installers. The version downloaded will only allow
the DSC options available in your country. Some functions described below may not be
available to you.
Save the file to your computer and open it to run the installer. Follow the instructions on
screen.
4.2.2 Configuration
Run the application from the desktop icon. The screen
shown here will appear.
Enter the nine digit MMSI of the vessel in the box provided
and select update beacon.
(In the USA, after the MMSI has been entered, a second box for a
group MMSI will appear. Enter your group MMSI here if applicable,
noting that group MMSI numbers should always start with a zero.)
On completion press the Update Device button to proceed.
A new screen with further instructions will appear.
To put the MOB1 into programming mode, slide the Arming
Slide down to the armed position and press the TEST/OFF
Key for at least fifteen seconds. The LED will start flashing
green. Release the key.