The Free On-line Dictionary of Computing (30 December 2018):

universal thunk

    A software mechanism allowing
   a Windows 3.1 application to call a 32-bit dynamically
   linked library (DLL) under Win32s.

   The Windows 3.1 application which wants to call an entry
   in a 32-bit DLL instead calls a corresponding entry in a
   16-bit DLL.  The programmer must also include code to
   detect whether the 32-bit DLL is loaded.  A 32-bit EXE
   loads the 32-bit DLL.

   See also Generic Thunk, Flat Thunk.

   ["Calling a Win32 DLL from a Windows 3.1 Application", Win32
   SDK Knowledge Base, Article ID Q97785].

   [Better explanation?]

   (1997-10-11)