In FB03, suppose you bring up a document and then click the header button.
It shows you a WE Goods Receipt pop-up with a reference key number/year.
Furthermore, if you take this reference key number into GLPCA, you will see that it appears as the GLPCA refdocnr.
So, since FB03 knows how to link the BKPF belnr to the reference key number, there must be a table with the link between these two numbers.
And therefore, you should be able to use the table to navigate from a refdocnr in GLPCA to a BKPF doc number (when they are not the same.)
But what is this table? That is, what is the table that links a BKPF belnr to a "header" reference key number that is the same as the refdocnr in GLPCA ????