This is a ring for mounting a lens onto the Chinese combination Thermal/IR monocular/optic known as the "MV265" on Alibaba and Temu. This is an incredibly capable device and only costs 290 dollars plus shipping from china. The only downside to this device is that it does not come with a magnifying lens so, naturally I fixed that.

Monocular:
https://www.alibaba.com/product-detail/Hot-Sellers-MV265-7X-Lens-9mm_1601288508520.html?spm=a2747.product_manager.0.0.4e5471d2I1xyNH
Lens:
https://www.amazon.com/gp/product/B01F9KXRX2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

It is intentionally a tight fit. You will need to use a heat gun or a hair dryer to insert the lens and press the ring onto the optic. Once it cools it shouldn't move, or at least mine hasn't yet even after dropping it on the ground.