I would think the maker/manufacturer specs are the best evidence of blade length.
Knife makers do not make laws. You're wrong in your assumption that manufacturers of any product are aware of local, state and even federal laws or there are standards that are accepted.
Take Kershaw for example. Assisted blade open knives were sold in TX for years and assumed legal before case law threw someone in jail and made assisted open knives a switchblade.
Now, if you were the person who'd assumed Kershaw knew the laws, and got arrested, then in the opinion of the judge, a Kershaw knife was a switchblade you'd be convicted even though Kershaw thought their knives were not switchblades and it wouldn't be Kershaw's fault.
TX legislator, with the help of several knife lobbyists, changed the law to define assisted blade opening legal. Too late for the guy who got convicted.
http://www.texaschlforum.com/download/file.php?id=115
http://www.lexisone.com/lx1/caselaw...=eSDY.hUQa.UYGX.YdaS&searchFlag=y&l1loc=FCLOW