Hi... I have two Omega D2, one with the variable condenser and another with fixed condenser but with a factory filter slot above the condenser.
On both units, I actually use Ilford's below the lens filter bracket. Instead of attaching them to the lenses, I made a bracket and attached it to the lens plate that attaches to the bellows bracket. The consensus is that having filters below the lens makes no difference when compared to above the lens solution. I've been happy with my solution. It's really easy to swap filters without disturbing anything.