Guten abend, XS60. Wilkommen!
Use Weekendrider's diagram #34. His matches your later frame.
I have (2) xs1b. Attached here is:
1 - Part diagram for older 70-73 brake, spring is #36
2 - Picture of inside, to the left is the brake arm, to the right is the protruding frame post with notch.
Spring goes to hole in brake arm to that notch on the post
Looks like you removed your spring post...
3 - Picture from bottom of xs1b, spring hooked to that post.