Maybe you have an Erlen rack from the time they came with one bolt? Lost the spares that came with the next round? of just need longer bolts?
This is a full set, 2@M5, 2@M6 1@M7 and 1@M8. this should cover the bases for "Most" seat posts that people using an Erlen rack would encounter.
For reference, some common posts and their bolts: