We've had the debate over the B24 a few times - it is an ongoing debate.
Scott only lists the used value - and in italics. Because they don't have enough info on previous sales & therefore, value.
I contend that the stamp does not exist. Isfila does not list it. I have never seen this stamp. It should be a 6-pointed semi-postal overprint on design A19, WITHOUT a discount overprint. The version that has the discount overprint is common - that is stamp B26.