Unusual measurement. One of our 2.5" coils should reasonably be just shy of 18.5" free height. At least it would if in "new" condition.
It's possible that over time and use (looks like they've been installed?) the free-height may very well have changed as the springs settled.
Can't say for sure.
Hope that helps anyway. Sorry didn't have an exact answer for you.
Do you know how long they were installed, if the PO gave you that info?
Paul