This is a short stub KF-16 to 1/2 inch MALE BSPT Tapered Thread. This thread size is primarily used in Europe and Great Britain. This adapter converts BSPT tapered thread to a KF 16 flange which is more commonly used in the universal world wide vacuum market.

BSPT = British Standard Pipe Taper (tapered thread).

Adapter KF-16 to 1/2 in. Male BSPT, Stainless Steel
Flange Size: ISO-KF NW-16
Pipe Thread BSPT (British Standard Pipe Taper): 1/2 in. BSPT-Male
Common Dimensions: A = 1-3/16 inch (30mm), B = 13/16 inch (21mm), C = 7/16 inch (16mm),
D = 1-3/16 inch (30mm)