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Wow people. You don't go apply for a tax stamp on a receiver before you have a parts kit. That doesn't even make sense. You have to write the type of firearm you are making on the application form. A pile of Parts isn't illegal...
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From the ATF website
Here
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.
For the I ain't clicking no link crowd http://www.atf.gov/firearms/faq/national-firearms-act-short-barreled-rifles-shotguns.html
Seems pretty clear guidance to me, but your might be ok.
It's possible they are confused, they work for the ATF.