To be nit-picky, it's an invalidate not a flush. By flush we mean writing the value out to memory. Since L1 is typically write-back, not write-through, you can have stuff in L1 that is not in memory. And so if you want to read from the MPB (like getting a new message) we want to ensure that you get the new stuff, so we provide CL1INVMB. Invalidate for us means that ... well, the cache line is invalid and a read sends you to memory.
Yes, 0f 0a is the opcode for CL1INVMB. There really isn't a CL1FLUSHMB ... although yes, sometimes people do call it a flush.
Alright, so CL1FLUSHMB is just a poor name for the same function. Thanks for the info, Ted.