As the Castle does not allow photos (the above photos came from the AMA Newsletter), exactly what that third display case contained has been somewhat lost to time. The best we have is a photo of Mark Wilson standing in front of the case on page 219 of The Original Houdini Scrapbook.
But now I've stumbled on a sensational color image of the full display case on the stock photo website alamy.com. Unfortunately, the site charges $50 for limited website use, so I'm just going to refer you to THIS LINK where you can see the photo on the site itself (click it to enlarge).
Some of the items in the case have been spread around the Houdini room, so they are still on view. However, the photos of Houdini's Machpelah plot (which appears to be this recently discussed Oscar Teale photo) and the photo of Houdini and Dorothy Young I've not seen in the Castle. Too bad, because they appear to be originals and would offer much better resolution than the images we have today.
UPDATE: According to our friend Joe Fox, the items in the case belonged to the late Manny Weltman.