By screwing the union nut onto the connecting part, the ring cutter can be inserted into the hole cavity of the connecting part, and the hole cavity has a conical surface. The outside of the seal ring is provided with a ring reinforcement at the nut side. The annular stiffener forms a torus at the transition area towards the narrow annular part. The inclination of the torus formed by the reinforcing part of the ring cutter relative to the pipe axis is 45 ° - 90 °. When fully assembled, the axial length of the annular stiffener enables the transfer element arranged on the annular surface to transfer its own negative image structure or itself to the end face of the connecting part. The transfer element has a three-dimensional profile, and its negative image can be transferred to the surface of the connecting part. The negative image is imprinted on the surface of the connecting part by the pressure exerted by the assembler in a manner similar to that of the die.