The VST IDs are the same, but not all hosts use the ID to err, identify the plugin. Some use the actual plugin filename.
Up through the current release, I’ve used different names for plugins on Windows versus Mac, depending on the plugin technology.
For example, 1973 might be…
1973.dll for VST on Windows
1973VST.vst on for VST on OS X
1973AU.component for Audio Units on OS X
This was probably not a wise choice on my part, because it does make moving back and forth between platforms difficult. I’m trying to correct that for the next release, although it will cause backwards compatibility pain. I can’t win on this one.
For now, your best bet is to rename (or copy) the 1973VST.vst plugin on the Mac to be 1973.vst. At that point, I think that REAPER should find it okay.