This is a replacement eyefinder beauty ring for the Canon F1 and F1(n). I am not positive if it will fit on a Canon F1N (the model with the front grip). 

The ID model screws into the existing threads, it fits snug at 100% and very good at 99%. 
The OD model slips over the whole silver ring, it fits tight at 100% and slightly loose at 101%.

Canon F1 Eyefinder Beauty Ring

canon_eyefinder_beauty_ring_OD.STL

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Canon F1 Eyefinder Beauty Ring