Radial mount backplates are a good thing for .049 free flight model airplane engines. This one was made by Texas Timers back in 1997. It's new in the package--has never been used.
The instructions say that it's intended for use in AME or Stels .049 engines. The people who made the AME later made Norvel engines. You can probably use this backplate in a Norvel Big Mig .049, although I haven't tried it.
The flange has 12 predrilled holes in it. As Hank said, you'll only need to use three or four of the holes to mount your engine---but the fact that there are twelve predrilled makes it easy to mount the engine exactly as you want it.
If you fly 1/2A free flight, you need this mount.