To me, that's a gray area. I've had this discussion with some on this board before and there are many folks out there that will tell you that if you have the parts to assemble an SBR but have not filed a form 1, then you breaking the law. The discussion at the time was about having the components even within the same premises. Basically, inside your house.
My defense was, how is one supposed to have spare parts then (which is your very scenario)? Or what if one is acquiring parts for a new build.
Is either side of the argument wrong? I guess that would be for the ATF to decide. BUT, here is some black and white taken directly off of the ATFs FAQ.
Q: May a FFL or an individual legally possess the parts to manufacture an SBR or SBS as long as no firearms are actually assembled?
A FFL (Type-7 or Type-10) who pays the Special Occupational Tax (SOT) may possess parts required to assemble NFA firearms. A non-licensee or FFL who has not paid the SOT is required to register any NFA firearm via an ATF Form 1 (5320.1) prior to acquisition of the parts required to assemble such firearm.
Again, I would argue until the death if I had parts for a separate build that could be misconstrued as parts for an SBR. In your case, you do have a registered SBR so it may be a non issue. However, that spare stock could once again be misconstrued as a stock for your pistol. See the cat and mouse game there?