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Burke and Feduccia Paper: Question

I finally read Burke and Feduccia's paper on the
embryologic development of the bird manus.
One issue regarding how to address their most damning points:

They state that digit 4 in "all amniotes studied" (p. 666), "the
post axial condensation extends distally through the ulnare
into the precursors of distal carpal IV and metacarpal IV....
termed the 'primary axis'" (p. 667). The authors cite two
references regarding this claim, neither of which I have
read, so I don't know how this developmental pattern was
arrived at.
Burke and Feduccia then claim that this early axial development
along the line of the humerus to digit IV is also seen in bird
embryos, thereby pinning-down a known benchmark digit to compare
with other amniote hands.  This is an important point that they made.
Question: Is the amniote digit-IV always in line with the
early "primary axis" of development? Is this testable?
Could a short-period of in-amnio injections of dye
or Tetracycline during this crucially-short period
of development in turtles, crocs, and mammals identify what
digit this axial element actually becomes?  Or is this
axial trait plastic (sometimes in 3, sometimes in 4)?
Their pictures all seem to strongly suggest digit 4.