Reserve police or deputies can't carry them legally either.
First there's this,
RCW 9.41.250
Dangerous weapons—Penalty.
(1) Every person who:
(a) Manufactures, sells, or disposes of or possesses any instrument or weapon of the kind usually known as slung shot, sand club, or metal knuckles, or spring blade knife;
(b) Furtively carries with intent to conceal any dagger, dirk, pistol, or other dangerous weapon; or
(c) Uses any contrivance or device for suppressing the noise of any firearm unless the suppressor is legally registered and possessed in accordance with federal law,
is guilty of a gross misdemeanor punishable under chapter 9A.20 RCW.
(2) "Spring blade knife" means any knife, including a prototype, model, or other sample, with a blade that is automatically released by a spring mechanism or other mechanical device, or any knife having a blade which opens, or falls, or is ejected into position by the force of gravity, or by an outward, downward, or centrifugal thrust or movement. A knife that contains a spring, detent, or other mechanism designed to create a bias toward closure of the blade and that requires physical exertion applied to the blade by hand, wrist, or arm to overcome the bias toward closure to assist in opening the knife is not a spring blade knife.
It's a bit of a myth to say you can have one but not carry it. I don't know where that came from. Notice the word possess up there.
These are the exemptions Look at a couple things... especially military who think you can carry them anytime you want... same for cops and firefighters. Only only on official duty. No, you aren't on official duty 24/7. See if your CoC backs you if you decide to go to trial.
Also look at what cops can carry them... "general authority law enforcement officer." It then defines those as full time, fully compensated. So no reserve cops or deputies.
RCW 9.41.251
Dangerous weapons—Application of restrictions to law enforcement, firefighting, rescue, and military personnel.
(1) RCW 9.41.250 does not apply to:
(a) The possession or use of a spring blade knife by a general authority law enforcement officer, firefighter or rescue member, Washington state patrol officer, or military member, while the officer or member:
(i) Is on official duty; or
(ii) Is transporting a spring blade knife to or from the place where the knife is stored when the officer or member is not on official duty; or
(iii) Is storing a spring blade knife;
(b) The manufacture, sale, transportation, transfer, distribution, or possession of spring blade knives pursuant to contract with a general authority law enforcement agency, fire or rescue agency, Washington state patrol, or military service, or pursuant to a contract with another manufacturer or a commercial distributor of knives for use, sale, or other disposition by the manufacturer or commercial distributor;
(c) The manufacture, transportation, transfer, distribution, or possession of spring blade knives, with or without compensation and with or without a contract, solely for trial, test, or other provisional use for evaluation and assessment purposes, by a general authority law enforcement agency, fire or rescue agency, Washington state patrol, military service, or a manufacturer or commercial distributor of knives.
(2) For the purposes of this section:
(a) "Military member" means an active member of the United States military or naval forces, or a Washington national guard member called to active duty or during training.
(b) "General law enforcement agency" means any agency, department, or division of a municipal corporation, political subdivision, or other unit of local government of this state or any other state, and any agency, department, or division of any state government, having as its primary function the detection and apprehension of persons committing infractions or violating the traffic or criminal laws in general.
(c) "General law enforcement officer" means any person who is commissioned and employed by an employer on a full-time, fully compensated basis to enforce the criminal laws of the state of Washington generally. No person who is serving in a position that is basically clerical or secretarial in nature, or who is not commissioned shall be considered a law enforcement officer.
(d) "Fire or rescue agency" means any agency, department, or division of a municipal corporation, political subdivision, or other unit of local government of this state or any other state, and any agency, department, or division of any state government, having as its primary function the prevention, control, or extinguishment of fire or provision of emergency medical services or rescue actions for persons.
(e) "Firefighter or rescue member" means any person who is serving on a full-time, fully compensated basis as a member of a fire or rescue agency to prevent, control, or extinguish fire or provide emergency medical services or rescue actions for persons. No person who is serving in a position that is basically clerical or secretarial in nature shall be considered a firefighter or rescue member.
(f) "Military service" means the active, reserve, or national guard components of the United States military, including the army, navy, air force, marines, and coast guard.