Fowler cylinder bore guage?

http://www.ebay.com/itm/New-Britain...xB4PBoOAoDTWU4SgJJ0yo%3D&orig_cvip=true&rt=nc

Just purchased this for $31.00 with shipping off EBAY. Not sure if it can measure the bit under 4" for the bore but I think I can make use of it. Thinking about what to use as an accuracy standard. Maybe a 4" bearing race?