The crotch black walnut used to stock the higher grades was the material is short supply and subjected to the salt-cure; high-grade shotguns as well as rifles were the recipients of the fancier stocks and of course the rusting problems. Flitches that weren't stickered in salt (more easily available straight-grained examples) were used to stock field-grade guns which then didn't exhibit the problem.
What was intended as a dessicant became a briny bath for many of the high-grade blanks. There has been conjecture that blanks stickered near the top of the stack were not subjected to the briny condensate which "pickled" those lower down and so absorbed less salt in solution. If this was the case, then "top of the heap" is a literal and physically accurate description of the wood on unaffected guns.
jack
