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

BSPP = British Standard Pipe Parallel (straight thread) and is often noted G, for example, G(1/2).

Adapter KF-16 Flange to 1/2 inch Male G BSPP Thread (British Standard Pipe Parallel), Stainless Steel
Common Dimensions:
A = 1-3/16 inch (30mm)
B = 13/16 inch (21mm)
C = 5/8 inch (16mm)
D = 1-3/16 inch (30mm)