This Stihl part number 1118-850-4200 is an Adapter Flange for Vacuum/Pressure Testing (used to test crankcase seals for leaks).
Because it is a workshop service tool designed to block off and seal the intake manifold or cylinder port during a leak-down test, it features broad cross-compatibility. It fits a large range of both classic and modern Stihl chainsaws, as well as several trimmers and blowers, etc.
Please note: Although it is highly versatile, there are a few specific models it will not fit.