Firstly you should know the ANSI ASME B16.47 Ser.A is similar with ASME MSS SP44, Ser.B similar with API 605 Flange. The short answer is Series A tend to be larger, heavier, have fewer bolt holes, and cost more. The longer answer is these were competing specifications that where brought together in B16.47 as is put forth in a note from our catalog: ASME has incorporated most of the MSS-SP44 specification into B16.47 Series A and most of the API 605 Specification into B16.47 series B. They have also added Blinds to these two specs. You may note that these specs include only Weld Necks and Blinds. For Slip-On’s over 24�you must refer to either Industry Standard (which are not automatically covered by ASME) or Boiler Code Flanges.