I had a look at the metal coloured rings on my two 1c enlargers, indeed they are 0.5mm. It does look like Leitz made these, as they are not simple "hard edged". These have rounded edges on the sides, typically Leitz when it comes to enlarger parts that are on the outsides of the enlarger. Apart from the two wooden easels I mention before, I also have their latest 30X40cm easel (12X16) which is white formica and comes without the clamping system. From the bottom of the rubber feet to the top surface of the board, it is 30mm . . .
In addition to these 2 bladed Leitz easels I also use two RRB easels, one small and one for 50X60cm (20X24). These are also 2 bladed and excellent. Their thickness is less than the Leitz easel, but I don't know their exact sizes. You see, it never mattered to me, without or with rings, or the thickness of the easels. Everything works fine and gets a lot of use.
Patrick, it is indeed so that you must be careful when placing the paper. When I have placed it under the stoppers, I always push the whole sheet against the left stopper before closing. Works almost all the time. In addition, after pressing the prints flat, I always trim the four sides of the print by about 5mm. This takes out the tension of the paper and gives better flatness. (this is about fiber paper). This takes care of any mistake in placing . . .