said by Hall:A person at the big-box store (please, comments about the reliability of store staff aren't needed nor desired) said that for a number of years, these plates are supposed to be "universal" for exactly the situation I'm running into.
The mounting plate for the BRK 4120 is standard, that is to say it will match almost any box it is to be mounted to and all powered smoke detectors nowadays have bases included that will match the pattern of the box the BRK 4120 was mounted to as such they are "universal"