Follow along with this...
The COPO's were L78's with the SS nomenclature and L78 engine & CID ornaments deleted, along with adding the BE axle, HD cooling, ZL2, & L72.
Because the COPO's were L78's with SS equipment removed could this account for the remaining cars?
Could the extras 1377 have been the COPO's which were ordered out and began as Z27 L78, even the first 50 Yenko's were X66 cars, so they got gills, black tail pans, vertical trim on tail lights, yet no SS emblems.