I have a pair of Beard two blade easels for 12x16 and 16x20. They are both fine robust pieces of kit. I had a disaster with my 12x16 one when i got careless and caught my sleeve on one of the arms bending it quite severely. Beard sold me a replacement set of arms at a very reasonable price. Top and left margins are adjustable from 1/4 to 2 inches in 1/4 inch increments. My models both have screw locks on the arms so when they are set they do not drift.
In my opinion they are one of the finest two blade easels you can buy. But they are very expensive. They will last forever.