I just called them. They don't sell just the shackles - which I thought was silly. Why not sell replacement parts?

Anyhow, the 5" and the 9" shackles are interchangeable and can be keyed alike. I ordered a 9" key alike to my others as well.

In general, the 5" works pretty well with bike racks and I think the 9" would work better for signs, sign posts, and parking meters for road bikes. I'd get the 9" for mountain bikes. Yesterday, I took my mountain bike out for a bit and it was pretty tight with the 5" shackle, the 9" would have been much better.

J.