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Wolverine has the stripped uppers in stock. I've priced it out though and building up an upper from parts costs a lot more than it used to cost to just buy a complete upper when they were available ($1k). I imagine there are quite a few spare lowers floating around from guys getting the Spec180 lowers but uppers never seem to be available.
I don't know for sure. I assume they are compatible. I'll have to dig through all my parts, but I know it's not a 100% complete upper. Missing some little doo-dads, I think.
I always intended to build an SA80 style lower for it but life, business, and other projects seem to take up all my time.