FITS IN SECOND SLOT!
This is the Long filament Guide for the Prusa Injection Molded Double Spool Holder. (Mk3+)
Please consider using the C Clamp to maintain Spool location.
https://www.printables.com/model/1982-c-clamp-for-retaining-spool-on-spool-holder
The author marked this model as their own original creation.