Yes, there is a difference and the difference can be quite significant. OEM (original equipment manufacturer) parts are designed specifically for that equipment.
The materials are selected to meet code requirements, seals are tested and qualified based on the expected operating conditions, and dimensions, surface finishes, and tolerances are based on a detailed analysis and test results of the entire design assembly.
"Duplicate" parts that appear to be the same can in fact provide unreliable, unsafe, and unacceptable performance.
Always use genuine OEM parts, particularly in applications in which steam pressure must be contained.