KHB seems to indicate that you need the shorter, 2 3/4 inch cone (421-101), along with the necessary associated lens disc: http://www.khbphotografix.com/omega/Enlargers/D2.htm
Hope this helps you in your search.
Capt, I have an extra 2 3/4" cone with the lens disk. The hole in the disk is 1 11/16".
If this would solve the problem, e-mail me at : Fyrwnrdy@yahoo.com
I have a 2 3/4 cone, a 4 3/4 inch cone and 4 flat plates (421-100) and 0ne flat plate with a 32mm hole ( came with a Wollensak 101mm f4.5)
For mailing costs from Canada.