A spool that has one removable flange, enabling a roll of film to be placed on it or removed from it without rewinding. 1
https://www.nfsa.gov.au/sites/default/files/uploads/2010/08/25/split-spool.gif
The split spool normally takes a plastic core that is removed after winding.
References
1British Standard Glossary of Terms used in the Motion Picture Industry.