The best you might expect is via word of mouth from the PO and, maybe..who he bought it from, etc.
It's a very rare instances that the unit will comes with the "original sales" papers.
For instance, on my 84, I have all the original papers from the sales/factory order/options, etc..I got lucky~(I am the second owner.)
The factory has not been very helpful in providing any records for me on my past search(64GT)..Your experiences with the 51FC is, I fear, rather doubtful.
First, it was made in CA and, that facility is LONG SINCE GONE.
If you get lucky, let us all know~!
Congrats on the 51FC