I have an 18 Litre (Canadian model) Presto canner with a similar looking weight. This canner was my Mom's in the 70s and 80s. My weight comes apart into 3 pieces:
-the "stem" which is a 5 lb weight meant for pressure cooking
-the bottom ring which added to the stem makes a 10 lb weight for canning
-and the middle ring which added to the other two parts makes a 15 lb weight for canning
It's probably just stuck together from age. Whatever you do, DO NOT reduce the time required for processing if you use the full 15 lb weight!
Your manual (if you don't have one you can download one from the Presto website) will tell you how to adjust PRESSURE according to your ALTITUDE, but the processing TIMES remain the same. I'm at 3500' so I use the 15 lb weight and process for the same time that someone at a lower altitude would process using the 10 lb weight.
If you don't need the full 15 lbs pressure, I would recommend getting a new weight from Presto or really working that thing apart (maybe some WD-40 might help). Or dropping it on a hard surface a few times.