Here you go - this is a typical rack for 250 - 500lb bombs. It's pretty much the same idea of the 1000lb-ers. The part you have is one of the 'grabs' that straddles the bomb in the picture. The screws were adjusted to keep the bomb stable in the bomb bay. The piece in the centre of the piece you have was attached to the 'lug' on the top of the bomb and had an electric solenoid that would release the bomb when the bomb release button was pressed by the bombardier or the 'jettison' button controlled by the pilot.
This type of rack was not unique to the Lancaster - it was used, in one form or another, in most WW2 RAF bombers - as well as the underwing bombs on lighter aircraft such as the Spitfire.