The block cipher is a main-stream means to provide confidentiality for data transmitted in insecure communication environments, and the S-box is an essential part in most of modern block cipher designs. In 1973, Feistel used a key selected S-box mechanism in his early block cipher designs. However, this key selected S-box mechanism did not get much attention after the DES block cipher was published in 1977. In this paper, we generalise Feistel's key selected S-box mechanism, compare it with existing close notions, investigate applying it in modern block cipher design, and demonstrate by designing the LBC example block cipher that the generalised key selected S-box mechanism can be useful for improving security and performance without intensifying computational effort and storage space in some application environments. Especially, we define the combined difference distribution table and the combined bias distribution table for the security of the generalised key selected S-box, and describe frameworks to analyse the security of a block cipher with a generalised key selected S-box against differential and linear cryptanalysis.