Starrett SA 103. - Square Steel Gage Block Accessory (Straight Jaw 12mm)
Straight jaws (sometimes referred to as outside extension jaws) are perhaps the most useful of the gage block accessories. These jaws allow gage blocks to be used to check INSIDE measuring instruments such as I.D. micrometers and calipers. These accessories extend the outside measuring surfaces of the gage blocks beyond the gage blocks themselves to form internal measuring surfaces with approximately the same tolerance as the gage blocks between the straight jaws.
See Data Sheet for additional details.