A BK1 is a backup file.
The BK1 file is created by the Autodesk range of products which includes the AutoCAD program. Files are initially created as BK files before being converted.
.bk1 File
Extension
Developer:
Autodesk
File category:
What are bk1 files & how to open them - Autodesk Backup File
File category:
Developer:
Autodesk