The F Virtual Window Manager is a virtual window manager for the X Window System. Originally a twm derivative, FVWM has evolved into a powerful and highly configurable environment for Unix-like systems.
Wikipedia reference: https://en.wikipedia.org/wiki/FVWM

An early Puppy version: http://murga-linux.com/puppy/viewtopic.php?search_id=596753763&t=6018

Some early Pups featured this window manager.