I'm reviewing a customer spec that is requesting the use of a Navships UT cal block, for flaw depth verification following calibration with an IIW block.  I don't work with any of the Navships specifications, but have looked at catalogs this evening.  I find physical block size at 12" X 3" X 1.25", hole size at 3/64, and looking at a catalog photograph only it appears the first hole is 1" from the edge, the next 5 holes are on 2" centers, and there is 1" from the last hole to the edge of the block.  Is this the correct spacing (center to center) of the holes?

Please excuse my ignorance, but am I correct that this block is used for shear wave calibration? , and not a straight beam block? And although the customer spec references "verification of depth" only,.... how is this block typically used?  Can someone give me a summary of the calibration procedure for the block's intended use?