They gave Kirby back all the originals they had left in their warehouse; unfortunately the warehouse had been systematically looted for many years, and Kirby received only a fraction of the more than 10,000 pages he drew in the 1960s.
All the really good stuff, including all the covers and the key issue art (Fantastic Four # 1, Amazing Fantasy # 15, Spider-Man # 1, Avengers # 1 and 4, X-Men # 1, Hulk # 1, etc., etc.), had been looted a long time ago; In the current market the original covers to these issues would be in the half-million dollar range. Mind-boggling.
While they may have been “looted” or given out as gifts by the publisher over the years, as with other stolen property, when it comes up at auction, it could still revert to the estate.