Plain Forwarder DLLs
Plain Forwarder DLLs contain no code but just references to other DLLs which actually implement the function. The loader that loads the executable that in turn loads the forwarder DLL will replace the entry points in the executable not by the entry point of the forwarder DLL but rather by the entry point of the DLL the forwarder DLL refers to (after all that is the purpose of a forwarder DLL).
On Presentation Manager this happens (possible) because the designers decided to stuff everything into a single DLL "PMMERGE.DLL". But at that point in time there where multiple applications around that referred to file "PMWIN.DLL" (and others like "PMDRAG.DLL" for example) instead. In order to ensure that those applications would continue to work, "PMWIN.DLL" was made a forwarder DLL.
By the way: libc also follows this concept, libc*.dll are all forwarder DLLs except for the latest version that you have installed. You can tell by the file size: the latest version of libc*.dll is about 1.3 MBytes in size while the others are only 150 kByte or less in size. At least they started doing this with version libc06.dll.