I am not quite sure what you are referring too?
Anyone know who might have information about military codes aft on the fuselage vs construction number?
As I understand it , as I am sure you also do, that the 51-15342 is the United States Serial Number. The 51 indicates the year the Serial no was allocated . in this case 1951. the 15342 is the consecutive number within the batch allocated for the Piper PA-18 ordered that year.
C/N 18-1342 is the Piper construction number for that build of aeroplane.
This aircraft I believe became the property of the Luftsportgruppe in Ravensburg in Apr 1971 as D-EAEQ. It was later registered LN-NAV 21st May 1975.