25CHF
TS-Optics Rotation Adapter, Filter Holder and Quick Coupling – T2 thread
Very convenient device that holds a filter and allows for 360 degrees rotation. It has a T2 (M42) thread on both sides.
More details here:
https://www.teleskop-express.de/en/adaptors-10/adapter-quick-changing-rotation-133/ts-optics-rotation-adapter-filter-holder-and-quick-coupling-t2-thread-12076