how about 21wb - it is crazy when threads go on and on and noone really knows what the other guy is really talking about.It's just an expression. It's more colloquial to type "push mower" than "self propelled mower" or "21 inch walk behind."
And lets be honest...anybody that takes the time to type out 21" Walk Behind is a tool...just call it a push mower. You're not writing a doctoral dissertation, it's an online message board. Everybody (or so I thought) knows that "push mower" doesn't necessarily mean push mower.