Cindy -
10-24 is the screw size - actually, it is the threads inside the jack nuts, and you buy matching screws. The 0.91" is probably the minimum screw length that will reach the threaded part of the jack nut. Of course you do not want to use a screw that is longer than the inside thickness of the wall.
To install the jack nut, you push the body of the nut through the hole in the wall, insert the screw into the center of the jack nut, and screw it down tightly. This collapses the strappy center part of the jack nut body, pulling the far end of the jack nut up tight against the inside of the wall, where it remains. Although you can remove the screw from the jack nut, it is not a good idea to do it repeatedly, and as you said, you don't really want to do it just to mount/dismount the shelves. So before you initially insert the screw into the jack nut, you need to run it through one half of a two-piece flush hanger hook or bracket. This piece will then be held against the wall by the head of the jack nut. It is this bracket that will hold your removable shelves. If your TM has one of the hanging cabinets (over-sink or over-stove), they hang from the kind of two-piece hanger you are looking for.
These flush hangers are readily available in any number of places. For example, Amazon has a number of them - enter "flush hanger" in the Search box. You can spend more money by going to Lowe's (Item #334848 Model #121158 - the picture is bad) or Ace Hardware (same crummy picture, Item no. 5329958 Model 121158). I'm sure Keith Wire can recommend some sources.
Bill
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