Good Used Seat Bracket
In the USA this is a common bracket that is attached to the back of the seat and then slips into the seat suspension assembly.
Center to Center is 3.5" wide and 1.75" top to bottom
Nuffield 3/45, 4/65
Leyland 344, 384, 245, 253, 255, 262, 270, 272, 282, 462, 472, 482