Double headed concrete nails. Easy to pull out, even years later.
The exceptionally hard duplex concrete nails don't break, and you should be able to get the hammer under the head anytime in the next 10 years.
The real hard concrete nails seem to be difficult to find in duplex...
Perhaps this would work: http://homerepair.about.com/od/interiorhomerepair/ss/nails_types_5.htm
What are those nails that the Plastic Tree Inspection Records are held on by? They have a protective rubber washer under the head and are super light. Its lighter than aluminium. Im guessing they were designed to be non-invasive.