Baader VariLock 29, lockable T-2 ExtensionTube 20-29mm with spanner tool (T-2 part #25Y)

Lockable T-2 extension tube, adjustable 20-29 mm with clamping
Telescope-side connection dimension: T-2 (M42 x 0.75), Eyepiece connection dimension: Optional T-2 (M42 x 0.75) or T-2 Ring Schwalbe 52 x 6.5mm optical length: 20-29 mm continuously adjustable over millimeter scale
Outer diameter: 50.8mm / 2 "inside diameter: with eyepiece side T-2 thread insert = 38mm with 52mm dovetail eyepiece sided = 41mm