Hi Greybeard,
The confusion on the Cooper's weight comes from the fact that the bomb's case weight was 20 lbs., but when loaded with explosive and a fuse (less the protective cap) came to approximately 25 lbs..
The bomb was an assembly of component parts.
Btw, the bomb carrier used for the Cooper is called the "20 lb. Carrier", and could also drop 20 lb. "Hales", "R.L", and "Incendiary Carcass" bombs too.