This is the 25.5 mm Infrared and UV Blocking Filter (39) from Heliopan. According to Kodak a BG39 has 85.2% transmittance at 400nm and 17.8% at 700nm. It is designed for use with black and white and color films that are sensitive to certain values of infrared and UV light not visible to the human eye. Using traditional contrast filters such as a yellow 12, red 25 or green 15 will produce a color shift which may not be desired in the final image. Greens may become blue, red becomes green and yellow becomes blue/green and infrared becomes red.
