Hi, the FJM's are for the older series of machines ie S512. I have not purchased FJM bags in ages so someone correct me if I'm wrong but the FJM bags are smaller from memory.
I think the compartment dimensions are different and there may be reduced airflow as the bag gets full.
Personally I just get boxes of 16 GN bags at a time and can use them in my S512 and my newer S5. The fitting is the same for FJM, GN and U Bags.
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