I did a bit of research this afternoon and listed the chronological changes to 9.41.050, carrying firearms.
At the bottom of the RCW there is a list of changes made since 1935. I have attached a PDF of the changes listed for each session. I did not see an open carry restriction but there is something interesting added in 1996.
>(7) Any city, town, or county may enact an ordinance to exempt itself from the prohibition of subsection (4) of this section.<
Subsection 4 starts...
(4) Except as otherwise provided in this chapter, no person may carry a firearm unless it is unloaded and enclosed in an opaque case or secure wrapper or the person is:
(a) Licensed under RCW 9.41.070 to carry a concealed pistol;
(b) In attendance at a hunter's safety course or a firearms safety course;..........(see attached pdf for more).
This was subsequently removed in 1997 and morphed into what we have today.
ETA: The following link is the legislation that got us to this.
It is interesting to note the governors veto message; Public safety demands stringent control over these dangerous weapons, with few and carefully tailored exceptions.
And the governors agenda, page 3.