I am presuming with the proper Form 4 approved by the ATF that in other states a person to person(trust to trust) transfer of a SBR would be lawful.

With 594, there does not seem to be a way to accomplish this in WA - a dealer can't do a background check on a firearm if it isn't in it's inventory and to get a SBR into a dealer inventory it would have to be transferred there with the right forms.

Does 41F solve this problem by producing a background check on the buyers that the seller could use to satisfy the legal requirement?

Have you contacted an SOT/FFL to see how they would handle this?

I don't really know much about NFA firearms, but a dealer definitely can do a background check on a normal firearm which is not in the inventory. I do it all the time - if people don't have a CPL we encourage them to come before the handgun is here so by the time it ships they are at least part of the way through the check. For p2p transfers we always do background check before taking the gun on the books.

Are things different for NFA firearms?

With every form 4 NFA purchase that I made I have had a 4473 run on me as a trustee of the trust.

By proxy - Yes - this came up in conversations with both WCA and the ATF while I was trying to determine how to transfer a silencer person to person if a background check applied. That topic was mired in grey area between Federal and State definition of a 'firearm' (feds consider a silencer a firearm, 594 does not). While the final answer for a suppressor was 'no' since it's excluded from 594, a SBR has no such exclusion as far as I know.

Based on the conversations mentioned and observation how 2 other Bellevue FFL's operate I was working under the impression that a firearm had to be on the books before a (4473)NICS check could be completed, but if that is not the case practically as Sergey mentions than there would be a non-issue here.

My presumption was that the only silver lining of 41F might be that you would have evidence of a background check existed as a function of a complete Form 4 in support of a private party transfer.

Well, yes. One would assume that (After 41f) a received tax stamp is akin to a background check, but there are no exemptions for that. A letter to the AG from a State Agency should help clear it up.

More evidence of how poorly written 594 is?

I suspect that with the future proliferation of SBR's in WA, expected party to party (person or trust) sales that this will need some legal attention either with another "align WA law to Federal law" or hopefully a 594 fix sometime this decade.

Hard to blame 594 on this. Lets be honest this is something even those in the firearms industry aren't 100% on since definitions don't line up. Plus its not really something that happens ALL that often nationally either. Not many people want to buy something then leave it with another person for 6+ months lol. And no I don't like 594 at all in any form. But bills cant always forsee everything....look at the SBR bill that had to be fixed

Hopefully the OP can get an answer though!