First: The chassis style determines whether the following applies (disclaimer ;-)

Second : The thumbscrews typically mount the lensmounts to the
lens cones, but some mounts used a machine screw to do so. Some cones
were acually flat boards or plates (and were square).

Cones and plates attach to the enlarger by sliding them into  the grooves
that have a spring tension device, the grooves are located at the bottom of the
bellows. This applies to chassis's that have a square bellows opposed to a tapered
bellows like the newer D5 chassis.

The lenses mount on the round flat lensboards.

The lensboards mount to the lens cones using either knurled screws or
machine screws.  Since the D2 bellows is so short the cones for different
lenses are different sizes, longer cones for longer focal length lenses.
Harry Taylor of Classic Enlargers says that 105mm or shorter lenses are
mounted on flat boards but I have cones made for my shorter lenses, too.
Find an online copy of the D-2 setup guide if you don't have one in hand.
It will aid in sorting out which lens uses which lens mount plate and cone.

Any machine shop can make new lensboards, good if you have odd-ball optics
that don't use Leica thread.  Recessed lensboards also exist for short focal
length lenses:  40mm, 38mm, 35mm, 28mm, 25mm, these lenses are useful for
larger magnification when printing half-frame 35mm, 110 and subminiature
formats.  Harry Taylor http://www.classic-enlargers.com/ and KBH
http://www.khbphotografix.com/omega/ have parts.  Harry actually has new
parts made for him.

D-2 and D-II cones mount with a twist.  You push up against spring tension
and twist to mount.  Twist the other way to remove.  D-3 and D-4 cones push
straight in.  Recessed lensboards won't work on a D-3 or D-4.