Could be - I was not there for that one but, I'll have to try to ask one or two of the designers of the the 11 FTN compiler back end when I see them next who was. Rich or Dave might know/remember.   That said, some of the folks on the simh mailing list might also remember like Tim or Bob.

That said, IIRC the PDP-11 on chip did not support it, so I suspect the FTN compiler did not use it.  I also know that in the past, when we had talked about some of the silly instructions that were created in different ISPs during lunch conversations, I have specific memories of some of the compiler folks saying they could never figure out PDP-11's MARK either - hence my comment about few did know how to put it to good use.