It fits the silver b/c and the 1000’s series. The issue is it will take more than 1/2 of the grill. Ergo you’d have to use the plancha by itself or get another grate that’s about 1” less wide than normal.
It’s a good price for a huge chunk of cast iron.
Home Depot has this on close out for $36 plus tax. Free Shipping
https://www.homedepot.com/p/Weber-Style-Cast-Iron-Plancha-for-Spirit-300-Gas-Grill-7579/204817365