Mr Klein replaced Mr. Grossman half way through series eight. He wasn't much different to Grossman, however, and both their absences were hardly noticed. It was also never established what position in the Men's Department Klein held (although it was presumed he was the Senior).
Although there wasn't much to Klein, he quite often seemed against relationships and occassionally hinted that he wished he'd never married in the first place. He was also against Mr. Humphries and Mrs. Slocombe flirting in "The Erotic Dreams of Mrs Slocombe".