Hello Tony

Does anyone know the proper name for the flange referred to as "wide
rectangular 8-hole"

If it's for WG16/WR90, that sounds like a precision flange. For a start you
could look at the Maury Microwave or Flann web sites. Unless you're very
lucky, you'll pay extremely large amounts of money for a transition to mate
with it ...

For real world use it's not that difficult to make a transition. Even if you
don't have access to metalworking facilities, it wouldn't be too frightening
cost-wise to get a local precision engineering firm to make one for you.
There's a very good, reliable, article by W1GHZ (on his website) which
contains cookbook information on WG-coax transitions for most of the microwave
amateur bands.

Vy 73

Chris
GW4DGU