The most common enlarger lens thread is 39mm. You need a flat panel for 6x6 enlargements using 75 or 80mm lenses. I'm not sure about 645. One of my flat panels is called Siriopla. The 25mm is called Lapla 25. Unfortunately there are no names on the original flat and recessed 39mm panels that came with the enlarger. I have seen the recessed one described elsewhere as Siriotub. The problem you find with the 50mm lens on a flat panel is that the bellows become too compressed before the image comes into focus. I have checked the instructions, but there is no mention of the different types of panels. It does confirm, however, that the one pair of condensers is used with all lenses, and that the column can be reversed for floor projection.