Simplex Bell Help

Hey everyone,
Just got 2 Simplex 2901-9361 Single Stroke Bells in the mail. They’re real nice, but I can’t figure out how to remove the gong to mount the bell itself. Instead of a hex bolt it uses a little silver circle in the middle. These bells are 6 inches I believe so you have to take the gong off the striker mechanism to mount them but I don’t know how to do that and don’t want to unscrew the wrong screws.

Thanks everyone!

Image (not the same model, but has the little silver circle thing) from Google Images:
https://www.buyfirealarmparts.com/shop/media/catalog/product/cache/1/image/5efbbbdf4778025c4bd71e93912f5b65/f/i/file_16_14.jpg

On the older model bells a 4 inch gong screws directly to a threaded stud on the mechanism. The larger gongs have a cast piece attached that has two “legs” that go into holes in the mechanism. There will be a screw on each side that holds the “legs.”