Do any of you know the size of the bolt that binds the seat to the seat post? I need the diameter and thread pitch.

I purchased a Brooks B72 and it has two rails on each side and required a spacer between the two rails. The spacer makes the bolt that comes on the bike too short for the job.

I think it is an M5 with a 1.0 pitch. The length I need is either a 50 or 55 mm, but dang if I can find one anywhere local.

Edit: I may have asked this already, but cannot find my post.