New belt. Thought I measured it right when I ordered it, but it was much too long... No screw.
Unpick the stitches (back of the belt):

Side view:

Almost there:

Thingy:

Done:

Select thread*:

Line up third hole with third hole a belt that is the right size:

Determine how much it needs to be shortened:

*) The thread was darker, but more importantly, thinner and weaker than the original. Off to the shop. To be continued.