This is a short stub KF-25 to 3/4 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 25 flange which is more commonly used in the universal world wide vacuum market.

BSPT = British Standard Pipe Taper (tapered thread).

Adapter KF-25 to 3/4 in. Male BSPT, Stainless Steel
Flange Size: ISO-KF NW-25
Pipe Thread BSPT (British Standard Pipe Taper): 3/4 in. BSPT-Male
Common Dimensions: A = 1-9/16 inch (40mm), B = 1 inch (26mm), C = 3/4 inch (20mm),
D = 1-3/16 inch (30mm)