Besler Adjust-A-Matic 16x20 Easel

[Smitty]

A while back I acquired a Besler 16x20 Adjust-A-Matic enlarging easel. Looks to be of great build quality but I am a bit confused regarding the adjustable blades, there are only 3 with the top part of the easel frame holding the paper. I had assumed that this easel was missing a blade but I have just seen another on ebay with the exact same configuration: -4-Blade-Bordered-Easel-/181868670753?hash=item2a5837af21:g:gGoAAOSwQPlV9aia

Can anyone shed any light on this?

Thanks, Steve