This is a short stub KF-25 to 1 inch MALE G BSPP. This thread size is primarily used in Europe and Great Britain. This adapter converts "G" BSPP thread to a KF 25 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).

Adapter KF-25 Flange to 1 inch Male BSPP Thread (British Standard Pipe Parallel), Stainless Steel
Common Dimensions:
A = 1-9/16 inch (40mm)
B = 1-1/4 inch (33mm)
C = 15/16 inch (24mm)
D = 1-3/16 inch (30mm)