Is that last one after the letter N a zero or an O? I have not seen that before on a Longchamp as an indicator of the build month.
As for the build date, it was likely build in late 1975. The registry from the Dutch importer shows car # 02073, 02083 and 02089 being registered from March through to May 1975 indicating that they were customer orders. Car # 02087 wasn't regsitered until October 1975, so was likely a showroom display car.
To clarify, this insn't the build date but there wasn't much time between building the cars, shipping and registry since most were custom ordered.
The next one is # 02217, again custom ordered and registered in March 1976.
I hope this helps you.
Chiel