The WPS for DECmate is very strange, even to OS/8 and COS. The disk format is weird; the character set is basically 6-bit with escape codes. However, there was a version of WPS for the Rainbow (incompatible with the DECmate) and I believe even one for MS-DOS on the PC.
The DECMate I looks like a VT100; the II is the spitting image of a Rainbow; the III looks a little like a Rainbow that's been hitting the poutine too much--it's somewhat taller than the Rainbow.
There's also a somewhat-related DECStation that's stuffed into a VT52 case, but also runs WPS.