Just take out the optical block and the mount from MD Rokkor 50/2
The lens was only $20, all plastic construction even the both helicals.
(I bought this as a donor lens for this modification. Not a bad lens!)
MD 50/2 looks > http://www.pbase.com/kkawakami/image/140129485

LBH = Lens block holder, moves back and forth when you rotate focus ring
its inner diameter is 40mm, which is slightly larger than B28 helicoid base.