Per Strapac's 'Southern Pacific Historic Diesels Volume 4': " From an appearance standpoint, there was little ( other than paint style ) to distinguish the first SD - 9's from the last SD - 7's. EMD responded to customer complaints about rainwater leaking through the classification light doors high up on hood ends by redesigning them for inside access. "
I would agree it is cheaper to weld on mounts than cut & finish access doors, but it appears ( at least officially ) that EMD was improving the product. Strange thing then; when looking through the SPHD book I noticed that all but the 1st order of SD - 9's have a triangular access door under each numberboard on both ends. Was this an Espee option to retain outside access to the numberboard for train number indicators ??? These doors were retained after rebuilding; maybe for bulb access ???