The 650 US LPF-12.5 is a Longpass Filter, a type of optical element designed to transmit light with wavelengths longer than a specified value while blocking shorter wavelengths. This particular model, sourced from Iridian Spectral Technologies, has a catalog number of 650 US LPF and is available for purchase at https://www.iridian.ca/product/650-us-lpf/. The product link is currently valid and indicates that the item is in stock.
The filter is circular in shape with a diameter of 12.5 mm and a thickness of 3 mm. It is made from a material identified as UVFS MAL-1965. The filter's primary function is to cut on at 650 nm, meaning it begins to transmit light at this wavelength, and has a cut off of 0 nm, indicating it blocks all wavelengths shorter than this.
The filter's coating is identical on both the front and back surfaces, both bearing the coating identifier IRR 650-US-LPF. The shape of the filter is described as a Thin Lens, suggesting it may have lens-like properties.