It seems that the conversion table was made for a larger set, so it may not be correct for the one you have. That table also indicates that the focal lengths of the cells were NOT constant, since my No.7 cell is 360mm and the one in the table is 250mm.
As you might be able to see from the picture, my Hermagis Trousse Aplanatique has a kind of bayonet fitting - you only have to rotate the cell a fraction of a turn to screw it tight, the threads are removed in three sectors.
Mine also has three cells marked 7, 8 and 9, while the fourth cell is marked Somethingorother (I'm at work now) 210mm f:6.8.
Yes, I mount it on my 810 camera, 4 elements can combine 6 combinations, from 210mm to 310mm. For single cell, how about quality when shoot photo? And is the No.8 and No.9 hard to find?