Here's a possible route:
In the flat, (under a Mirror modifier) sort the topology as much as possible. Insert an extra vertex in the outside diagonal, scale to make rectangular quads.
Extrude to thickness. When extruding under a Mirror, you have to delete internal faces on the axis. I H hide a face for access,select one of the faces to go, ShiftG select coplanar, and X delete.
I inset the top faces, with 'Boundary' unchecked, laying the foundation for the flange. Cut to correct the mitre on the left to crossed edges, and E extrude the flange, not all the way round.
ShiftH Hide all but the top faces of the flange, and use Linear proportional editing to lower the right-hand end.AltH unhide and adjust. Delete screw-hole top and bottom, Bridge Edge Loops to connect, scale-adjust and maybe extrude bottom at this stage, to block out the countersink.
If we want an extra edge to round the tapered end of the flange, we can introduce 2 more edge-loops on either side of it. (I made a mistake here, it would have been easier to make 2 on the inside) In order not to disrupt the curvature in the screw-hole, we can introduce another loop around it to accommodate a 3 to 1 'kite' reduction of the loops, top and bottom, before they reach the hole.
All done bar Bevel and Subdivision, working under those, to GZ edges to curve the floor-level, E extrude out the back plates, and tighten things up with some holding loops.