Trying to put my thoughts down on this.
A stripped lower in MA is not a firearm and shouldn't have an FA10. Some shops do when you purchase but seems go be overkill. It needs to be registered when it can fire a round. Now it's a firearm in the eyes of MA.
With this new guidance, anything before yesterday is okay (for now). So, if you have a stripped lower that wasn't FA10d then I think you'll have more risk in building it later even if you owned it before (hopefully you have a receipt, with date and serial to prove). If you registered it, then it can be built and possessed (for now).
Short story is from my take of you registered them then they're good to build. If you didn't, and have no paper trail, risky to build later and FA10. If you didn't and have a paper trail, less risky to build later and FA10. It's just so damn confusing. The guidance says assault weapons, but then says receivers are prohibited as assault weapons, even when they aren't firearms in MA. Does that mean as assault weapon doesn't even need to be a firearm anymore?