I think you're right - it must have been some sort of hide glue. The stick type disk adhesive was usually cleaned off when it was time to change the disk by holding a chunk of wood against the rotating disk. This wore off any abrasive disk residues. To really do it right, you needed to remove the disk sander's platten after the old disk was removed, warm the platten, apply the cement, then put the platten & disk in a screw press to ensure that the two were well bonded and flat (lumps were not good...). My pattern-maker uncle modified his disk sander so the disk was held on with 3 flat heat screws, which let him remove the platten easily. After he mounted a new disk, he'd use a utility knife to cut out the holes for the 3 screws, then he'd remount the disk on the machine. He usually had several disks ready to go, so he could switch from fine to coarse abrasive in a minute or two. I still have the disk press he built - since he was a pattern maker, it was all nicely finished cast iron and sized to hold a 12" diameter disk. It is totally useless for anything other than mounting sanding disks, but it is too good to throw away, but not good enough to keep.