I found listings saying these Rear Sight Screws were used on Springfield: 1868-1870 / .50-70 Trap Doors, 1855-1864 / .58 Cal Musket, possibly others to include Chiappa's 61 Springfield (Special?)
Picture is from this link: Rear Sight Base Screw - 61 Springfield | Comer's Gunworks LLC | Sikeston | 63801
https://cdn.filestackcontent.com/AUc...lQQeiKRkJmSZYZ
For Rear Sight bases like this:
https://www.ssfirearms.com/prodimages/giant/w22034.jpg
Hope every little bit helps.... :dunno: