Shane,
It takes the metal Class 66 bobbins. I get mine from Sew-Classic. Jenny's are really nice. You can use the plastic ones but they vary in quality too.
A rule of thumb is: metal bobbin case = metal bobbins; plastic bobbin case like the later ones have = plastic bobbins.
My wife doesn't care, she just uses what ever is handy. Her 538 "should" use plastic bobbins but she's been using both in it since the 70s when she got it. No damage I can see.
I see your assistant is taking a nap on the job
. She's so cute.
Joe