I have a Greist 8 pack of plastic cams in my collection. Although not branded as Singer they are identical to those Singer sold and will fit the Singer Button Hollers.
It comes with:
1" Straight
15/16" Straight
7/8" Keyhole
7/8" Straight
5/8" Keyhole
1/2" Straight
3/8" Straight
Eyelet
Joe