Fastener "head stampings" or ID numbers/letters

As far as I can tell the embossed 7 and 9 are there for quick bolt identification during assembly on the production line.  They have no meaning regarding bolt strength.  I ahve taken original fasteners, inspected them, ran them through a tap or die, media blasted, tumbled, and had them replated.  

From Wick Humble's standard reference, 'How to Restore Your Datsun Z-Car'"

 

"On ISO fasteners, the grade is indicated by its number - 4, 5, 9 and so forth - on the bolt head.  But this doesn't mean that a bolt marked 5 is equivalent to SAE Grade 5.  Unfortunately, the ISO number indicates a torque capacity lower than the SAE number -- a negative safety margin if you mix them up.  For instance, the ISO Grade 7 is the equivalent to approximately an SAE Grade 5 -- for a 14mm bolt, Nissan's torque spec on this bolt is 56-76 lb-ft.  ISO Grade 9, the highest number I've found on any Z-car bolt, is torqued 80-108 lb-ft if it's a 14mm bolt.  At the other end of the spectrum, anything ISO Grade 4 -- as are many of the 10mm Phillips head bolts found on a Z -- is comparable to unmarked 'hardware-store' grade bolts.  |That means they could conceivably be snapped off with an overenthusiastic twist on a Phillips screwdriver."

I found the following data to help clarify the bolt head marking subject:

This document can be found here, thanks Chas.

http://www.classiczcars.com/topic/49520-bolt-sizes/