It was common for those seats to come loose. They were only a press fit. There was an oversize made but I'm not sure it ever made it to the parts file. I always fixed them with my own technique. That was to lay the seat in the gap formed by an almost closed vise. With a sharp punch and a sharp impact form a series of around 10 dimples in the area of interference. This is similar to knurling and expands the overall diameter. Drive the seat back into the carb bowl and get a new needle.
Bosch Master, ASE Master, L1
33 years MB technician