Durst made the Latub for most models of its enlargers, to allow a short focal length lens to be used. The big models, like the L184 for 8x10, used one with about an 8-inch outer diameter and would recess the lens 2 or 4 inches into the bellows. The big ones dont need a handle for the aperture, because they are big enough for your hand. The one you have is for the smaller models, like the L1200 or L1000 for 4x5, among others. Im not sure about all the smaller models, but the L1200 uses a Latub with a 28mm recess to focus a 50mm lens. There are even deeper recessed ones. They are somewhat rare, so I would guess the value of the one you have at around $30 or so
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