Pretty much everyone and their mother had a lounge on C/D at one point.

My memory is a little rusty, but I recall in 2001 BA/AA/TW all had lounges on D, in addition to UA. By that time, I think everyone had moved to B when it opened except for AC/UA/BA/AA/LH/TW/OS. I recall a couple of other airlines who UA was partnered with using D at the time...I think BWIA and Spanair but they didn't have lounges. I also don't recall KLM haven't their own lounge at Dulles. LH never had their own lounge on C/D-before they moved to B I remember their lounge being in the Main Terminal building.

When B opened, I think ANA, VS (still there), AF (still there), and NW opened lounges on the concourse. Not sure what happened to the NW or ANA lounge space on B, or if they ever had one on C/D prior to moving to B. JAL also had a Sakura Lounge at one point but I don't know if it was ever on C/D or if it was in the Main Terminal.

The skinny black door with the wood-paneled windows reminds me of how Northwest used to style their World Clubs, so maybe that was their old lounge before they moved to B. The black tile is reminiscent of the Admirals Lounge on B that closed a few years ago (still intact with the AA logo just covered up with a sticker) so I am guessing that was an AA lounge.

Keep in mind that the concourses went through a bit of confusing renaming over the years. The present C/D was always referred to as C/D. For a while they were in fact two separate buildings connected by a covered walkway. I think later they added gates onto the walkway. The present Z gate area used to have two separate piers (the "South Concourse") and for a while they were called the M gates before being called the A gates.

The present A concourse opened in the 1990s under the name Concourse B. Once the regional wing was added in 2000/2001, it was called Concourse A, and the present Z gates were renamed to Concourse T, like ATL. Once MWAA began to expand Concourse B they renamed the eastern portion of the concourse (closest to the regional gates) to Concourse A, and the western was named Concourse B. I believe MWAA still was shuttling select flights directly to the plane using plane-mates up until that time period...I think the gates were called Concourse E which meant a hardstand gate IIRC.