A documentação do kernel Linux Arquivo obrigatório de bloqueio para o sistema operacional Linux tem alguns dos a história:
The System V mandatory locking scheme was intended to have as little impact as possible on existing user code. The scheme is based on marking individual files as candidates for mandatory locking, and using the existing fcntl()/lockf() interface for applying locks just as if they were normal, advisory locks.
A file is marked as a candidate for mandatory locking by setting the group-id bit in its file mode but removing the group-execute bit. This is an otherwise meaningless combination, and was chosen by the System V implementors so as not to break existing user programs.
(Mais especificamente, o livro Programação Avançada no Ambiente UNIX diz que foi feito no System V Release 3.)