I checked at allmusic.com and it looks like Rushent is a Producer/Engineer (like Birch) who seems to have gotten more in to working on punk/new wave stuff like Generation X, The Stranglers, Go-go's etc.
In checking on what they had for Birch, he had been working on Deep Purple albums in 1971 so he may have just been booked up with that or something. (?)
I would copy the links here for you but I tried copying some allmusic links to the PG board this morning and they didn't come across so I'd suggest just going to allmusic.com and type in MR & MB for the search and it'll list all the albums they worked on respectively if you're interested in seeing the full lists.
John