The F&G setup is used by Eureka and Sanitaire vacuums. The bags have a paper fill tube and are apparently far less satisfactory on a Kirby adapted to use them. It's far better to use the entire bag assembly from a Heritage II or Legend II. There is simply better and more efficient airflow through the wire reinforced plastic fill tube. This setup can also use the latest Kirby F-style HEPA bags for optimal efficiency. My Classic III sports a brick red Legend II bag with the original red Classic III bag topper. Another option, if you're good with a sewing machine, or know someone who is, is to put a full length zipper onto the Classic II bag and install the Heritage II style mini-em-tor and fill tube assembly into it.
I've also made the same modification to both of my orange bagged Heritage 1HD machines. One came to me with a dump bag, which I detest, so it got a complete gray Heritage II bag assembly until I eventually got hold of an orange bag and put the Heritage II fill tube and plastic mini-em-tor onto it. Later, I acquired a Heritage 84, which had been run without a filter bag, so I vacuumed it out and removed the metal em-tor and fill tube assembly to run the bag through the washer. After it dried, I was unsuccessful in reinserting the original fill tube assembly, and eventually just gave up and installed one from a Heritage II. Functionally, it's great; however, I wanted to use the original em-tor and fill tube assembly with the Type 2 bag adapter so that I could use up my supply of Type 2 bags before replacing that bag adapter with a Heritage II style one.