During the German occupation and until his deportation in December 1941, Klein worked on several ambitious chamber works consecutively. Some are complete, or finished enough to be performed, but there are a number of fragments which are too incomplete for realisation. The Divertimento was a work in progress from June 1939, and completed in April 1940. The almost-complete Duo for Violin and Viola in the Quarter-Tone System was embarked on in late December 1939, when he was staying in Přerov, and was worked upon certainly until late February. The Preludium for Viola Solo is dated 3 April, and May and June 1940 were taken up with the Three Songs for High Voice and Piano, Op. 1. He worked on the String Quartet, Op. 2, for a year, from August 1940 until the following August. The incomplete Duo for Violin and Cello is dated November 1941. The string pieces among this group of works are very much companion works, exploring similar language and demonstrating his increasing confidence in writing for these instruments.

The fair-copy manuscript of the Duo for Violin and Viola in the Quarter-Tone System carries the inscription ‘To Prof. Alois Hába in respect and devotion’, a fitting dedication for a work which draws on Hába's pioneering accomplishments in quarter-tone, or microtonal, music, and Klein's only music to employ Hába's system. His fair copy is of the first two movements only. The sketch of the third movement is reasonably detailed in terms of performance indications, although it replaces an aborted, crossed-out third movement, running to over 40 bars. In the absence of tempo and dynamics at its opening, the published edition of the Duo, edited by Milan Slavický and Vojtěch Saudek, proposes a maestoso and mezzo forte opening for the third movement. The aborted third movement opens Andante, with the instruments muted, and it can be argued that the extant third movement should follow this type of character, too, thereby providing some elegiac respite in an otherwise energetic work. The sketch of the fourth movement is also detailed, but is incomplete by its 34th bar. There is also an undated and incomplete Hába-inspired sketch with the title Fantasietta – Hába often used the title ‘Fantasy’ in his works – also for violin and viola in the quarter-tone system.