The Stevens Model 9478 was manufactured in 10, 12, and 20 gauge and 410 bore. It is a single shot break open shotgun with an external hammer and inertia firing pin. It was made from 1978 to 1985.
These are trade in prices, add 20% for retail.
Savage/Stevens does not publish serial number data for dates of manufacture. They used a letter code until 1970 and then discontinued it.