This is the V-Band turbine outlet weld flange for Borg Warner S200 and S300 turbos with the "4.21 Marmon" outlet. This billet 304 stainless steel weld flange allows a 3.5" OD tube to slip fit inside for welding, and will fit a 4" tube for welding on the outside.
Borg Warner decided to call V Band flanges and clamps by their original name from the early 1900s, Marmon or Marman (they have been spelled both ways). A Marmon flange simply means that there are two machined surfaces connected with a clamp. Borg Warner offers two style of marmon flanges on their housings, 3/4 and 1/2. 3/4 marmon means the flange has an angle on it and is not flat. A 1/2 marmon means the flange is flat. The 3/4 marmon flanges are common on the S200sx, S200sxe, S300sx and S300sxe factory T4 turbine housings. The 3/4 marmon is also used on the T6 S400 housings. The 1/2 marmon flanges are found on the T4 S400 and T6 S500 factory housings, as well as the S400 compressor outlet flanges on the standard Series 60 style housings.