tool in the form of a rectangular block with a round, concave depression in one side
Molding blocks are used to shape curves into flat materials, especially leather, by laying over top and hammering into the depression. Examples: heel seats of insoles, heel counters.
Compare closing block
Mamoru Anaboko Dimensions
It’s a 23.8 x 11.8, 4, 5 cm block with a 9 cm diameter hole of 2.1 cm depth and weighs around 700 g. — https://oneandawl.org/t/267/17