Original Huben regulator. Select the housing based on your K1 version. The model 2021-2022-2023's thread is asymmetrical, meaning the thread on each end is different. Model 2017-2018-2019-2020's threads are symmetrical, the thread size is the same on both ends.
NOTICE: Huben Airguns Shop cannot take responsibility for selecting the wrong housing for your K1. The standard return policy applies to this case, should you want to proceed in that direction.