It might be Creative Commons Public Domain or New BSD License (3-clause BSD License). Personally, I'd prefer the second one because it allows me to include your work into my project, but protects your copyright from being used in promotions.
Would you mind to provide a license? I'd like to include these assets into an open hardware project of mine. They'll be distributed both in a converted form and as images, but I'd like to provide a specific license anyway.
Would you mind to add a license you use to distribute this work? I'd like to buy this asset to use in an open hardware project of mine, but I'm not sure which license to use for this asset.