ATF Interpretation as rendered in ATF Ruling 2009-5 would seem to say that the person who modified the frame did so illegally. Without copying the whole ruling, which is relatively short, only four pages, here is a link.
https://www.atf.gov/file/55476/download
If you want to get down into the weeds, here are the references the ruling was based upon:
18 U.S.C. 923(i): IDENTIFICATION OF FIREARMS
26 U.S.C. 5842(a): IDENTIFICATION OF FIREARMS
27 CFR 478.92(a): IDENTIFICATION OF FIREARMS
27 CFR 478.92(a)(4)(i): ALTERNA
TE MEANS OF IDENTIFICATION
27 CFR 479.102(a): IDENTIFICATION OF FIREARMS
27 CFR 479.102(c): ALTERNATE
MEANS OF IDENTIFICATION
ATF authorizes licensed manufacturers
who perform a manufacturing process on
firearms for, or on behalf of, another license
d manufacturer not to place their serial
numbers and other required identification ma
rkings on the firearms, provided such
firearms already have been properly marked
with a serial number and other identifying
markings as required by
27 CFR 478.92(a) and 479.102(a) a
nd that all of the other
requirements stated in this ruling have been met.
Sometimes its best not to ask a question to which we probably know the answer? In my non-official opinion, the work performed on the frame was illegal and as such the firearm would be subject to confiscation and destruction as a defaced firearm.