No clear idea how to replace the bobbin--case Singer HD-110?

Took it out to clear a knot and it won't go back in. The book is no help at all. Unclear pix and reference points don't match anyway. Bobbin case won't seat. If I push it under the bar, the back pops up. I think everything else is right. I've had this machine for a few years and never had this problem before.