You can actually fit a Eureka or Sanitaire bag onto the Hoover bag horn assembly, giving you the right sized bag for the F&G (or if you're a purist, buy a new adaptor, bellows and exhaust tube and assemble a new horn assembly, saving the old bag for displaying the vac), that way you can get the more hygienic qualities of the F&G bag, and better airflow & filtration...
Plus the open-ended nature of C bags means you have to do some strange acrobatics to stop them spilling their contents when changing bags, the later Hoovers with the up & over gutterpipe filltube was a good solution, but ultimately the better solution was to do what everyone else was doing, and having them connect to a filltube through a card plate, pop it on, pop it off, no muss, no fuss, the F&G being an exception, and it has it's own drawbacks with the paper filltube being easily pierced if you pick up the wrong thing...
The C bags (or H1 in the UK) are just not good enough at filtration, you lose power almost immediately after fitting new, I even tried a genuine Hoover one in my Sanitaire once (just for fun, and I had no spare F&Gs at the time), and the power loss was immense after a few sweeps, I know Hoovers aren't as powerful as a Sanitaire so probably don't feel the loss as quickly, but, honestly, I think that using those bags in this day and age is just daft (but that's why I use trimmed down bags from my Victor's bag stock, it has microlining to improve airflow)... :\
