I do remember an autosport report that stated it as an FW20B - I cant remember if it was raced or not to be honest but 20B was used in post 1998 season or pre-1999 season testing prior to the FW21's launch.
There was an FW20B also which had an extended wheelbase - plus it featured a more pleasant to the eye Winfield livery - a simple wau to spot the FW20B is the front wing nose legs are at an angle and not straight.
there was also a FW16C IN 1994-95 Winter testing prior to the FW17s launch - best way to identify the FW16C Is its front wing had red end plates with segafrendo sponsorship
The fw11c was the test car for the Judd Engine and cam into existance when honda jumped ship and williams had to rusgh and find a new engine - they opted for Judd and had to make drastic mods to mfit it to the 11b chassis - hence FW11C