This is a Silicon Flat Window, model SI-W-12.7-2, offered by Alkor Technologies. The window has a base shape of Circular, with a diameter of 12.7 mm and a thickness of 2 mm. The material of the window is identified as "silicon sal vil 1957".
The window is designed to function as a Thin Lens, with a wavelength of 633 nm. It is uncoated, providing a clear, unaltered view of the light passing through it. The window's geometry and material make it suitable for a wide range of optical applications.
For more information about this product, you can visit the Alkor Technologies website at http://www.alkor.net/Silicon_windows.html. The link is currently valid and indicates that the product is in stock.