Rare early Japanese made HF-2 Hi Band Flanger.

Unique effect that flanges the upper octave only

From Effectsdatabase;
The HF-2 is combining the original signal with a delayed signal that is shifted one octave up to create a flanging effect a bit different but still similar to the BF-2. The effect may appear to be brighter and cleaner than the BF-2. The HF-2 may not be as powerful as the BF-2, but for hi frequency flanging it produces a very good result.