I believe the 623 is the 70s right side 340 manifold. It should have a 1 7/8" exit hole. The 623 was used opposite the larger 2 1/4" driver's side. I've never been able to figure out why Mopar would put one small and one large manifold on the same engine.
It has some value but not nearly as much as the 2 1/4", I sell the 623 for $90 -$120 depending on condition. Most people only want them if they're restoring a car and want it all original. Most everybody else would prefer a large 2 1/4" manifold on both sides of the engine.