My Schneider 150/4 Apo-Componon has a small extension cone that fits between the lens barrel and the rear cell. If you don't have this part, you might try ordering it from Schneider. I think it comes standard with the lens.
There are three ways of mounting this lens. One is with the cone, which is a direct fit in an Omega D lens cone in place of the small metal disk that usually holds the lens on the lensboard or the big cone. Another is with the 55mm threads. You can also unscrew the rear element from the barrel and attach the lens to a lensboard with a 39mm hole, using the rear element to hold it in place. I believe the elements are also a direct fit for a Copal 1 shutter, so that's a fourth option.