LCK file extension is for Lock file especially for database access control
Similar to LCK
3DMF - QuickDraw 3D Metafile QuickDraw3DS - Graphics Autodesk 3D Studio
3FX - 3D effects CorelChart
3gp - Video format compatible with mobile devices
3GR - VGA graphics driver Windows Video grabber
3T4 - Util3 Binary file converter to ASCII
4C$ - 4Cast/2 Datafile
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