Korolyev SK-6 / SK-7
The Korolyev SK-6 experimental two-seat, twin-boom glider of very compact design, made wide usage of Elektron magnesium alloy in the structure. Built in 1931 or early 1932, there is very little information about this glider. It never participated in any All-Union gliding contest and wasn’t among Korolev’s favorite designs. Only one photo remained which is identified as SK-6 glider.
It was designed (very similar project, anyway) in April 1931 as two-seat training aerobatic glider, but under the designation SK-7. The historian Georgy Vetrov guess it's the same design. But why Korolev changed the designation – it is another mystery.

Calculated, for SK-7 project from April 1931
Wing span: 10.0 m
Length: 5.45 m
Wing area: 17 sq.m
Flight weight: 230 kg
Gliding ratio: 15:1
Rate of sink: 0.9 m/s