Where is the 604A serial number

[Stormer]

I am considering purchasing a partially completed RV6A from my brother-in-law. The problem is that the wings have gone missing(long story). So I have to fit a new(or old) set of wings to the existing already built fuselage. The existing fuselage was built in 1992 sometime and back then they used a different machining technique. The company that makes the spars say I have to check the serial number from the center spar, part 604A, and if it ends in a NP or NA, it should be a universal fit for a new set of wings. So my question is: Where do you get this serial number? From paper work or is it stamped on the center spar 604A somewhere?