It's a high-quality aluminium screw-in adapter.
 
Technical data:
Thread 1: M20x1, 5
Thread 2: Dash 6 (9/16 - 18 UNF)
Material: Anodized aluminium
                                                    | Manufacturer: | Arlows | 
| Category: | METRIC THREAD | 
| Item number: | AR3499 | 
| Angle: | Straight | 
| Thread: | Dash 6 | 
| Product weight: | 0,10 Kg |