For all things Omega, there are two sources that you must have links to:

1) KHB, in Ontario, Canada; and
2) Harry Taylor's site, Classic Enlargers.

KHB is very good for Beseler too.

KHB has this invaluable lensboard reference:

http://www.khbphotografix.com/omega/LensMountD5.htm

Here is their main link:

http://www.khbphotografix.com/

Classic Enlargers has a site at:

http://www.classic-enlargers.com/

If in your travels, you come across an Omega 421-935 Extended Plate 42mm Hole
(previously 421-135) lensboard for a reasonable price, we should talk .

Matt