I love a mystery! Many of the “collector knowledgeable” nuances here, well beyond me.  Yet the most recently introduced element of “tang length” seeming perhaps the greatest ‘rifle specimen centered’ conundrum.  Such as differentiated from “provenance documentation”. 

Even IF extreme postulation of a factory short tang receiver replacement with serial number-accommodation corresponding to unit replaced; matching inletted wood included…   Surely such extraordinary instance arising to worthy of some official “notation” yet preserved?  All such, of course, necessarily ‘pre’ engraving work commencing. 

Last remark, as of at least a couple of decades ago, the quality-potential of counterfeit documents challenging most human analytic capabilities and ever more threatening.  Thus the arise of “security features” attendant to documents, etc. Questioned Document Examiners, more often ‘pinging’ on underlying media such as paper.  In the instant case, humans with power to recollect and verify their own expert opinions, as yet “pivotal” as to the subject matter “verified”. “Engraving attribution” as genuine alone, not necessarily explaining the “short tang tilt” unless addressed inclusively! 

I’m also looking forward to the personal interest & learning experience potential concerning this “authentication” issue!

Best! 

John