This is Rolleikin 2 that enables TLR camera to use not 120 roll film but 35mm film.
Rolleiflex, whose film pressure plate is switchable from 6x6cm format to 24x36mm format, can attach it, and also Rolleicord Va and Vb can attach it. (Refer to Rolleicord Vb.)
The first Rolleikin was introduced in 1932, which made it possible for Rollei TLR camera to use a color film, for there weren't a color 120 roll film yet in those days.
On the first Rolleikin, you had to exchange the rear cover for Rolleikin before attaching it to the camera (Refer to Rolleiflex V.), but it had been improved step by step and this, Rolleikin 2 was introduced afterwards.