I have a NT8B91 busy field lamp and the question is ... will this work on a M7324 set? It came off a M7310 and was working fine on that set.

Thanks in advance!

No it only works with the M7310 phone.
You would need to get a CAP for the M7324.

It was made to somewhat turn a 7310 into a 7324..or thats the conventional wisdom. Like it was said, if you need more, you need a CAP