Okay, let's clear a little confusion. There are two version of D-2. The basic D-2 has different condenser sets for each lens focal length -- the idea is that the condenser ought to focus the light source onto the lens, more or less. The D-2V has a "variable condenser" in which two main lenses are fixed, with a third movable and removable; it's used in different positions for different focal lengths, and removed entirely for 135-150 mm lenses used on 4x5.
If you have a D-2V (it'll have a door above the negative carrier with the condensers behind it), and the smaller third lens is missing, you're stuck with the light focused at the correct distance for 135-150 mm lenses -- but you only need the small lens, not a full set, and any concenser glass the correct diameter and focal length will work (it doesn't have to be from Simmon Omega). If you've got a D-2 (no V), and have only the longest "4x5" condenser set, then you'd need the additional condenser sets for whatever shorter lenses you might want to use -- 6x9 (105 mm), 6x6/6x7 (75-90 mm), and 35 mm (50 mm) will each have their own set with this setup (which is why the Variable Condenser version was created -- LOTS cheaper for the multi-format worker).
(edit: looks like someone else said pretty much the same thing at the same time -- happens around here)