X-Tech,
Your probably right. It's just a name. Not really a technical answer.
10 people would give 10 different answers.
And each year, with new cars produced, Junior Stock would change, along with the NHRA weight breaks and HP re-factoring

.
The 1966 NHRA rule book at least stated something,
B/Stock thru O/Stock, and B/SA thru G/SA. "Jr. Stock Eliminator".
A True Junior Stocker "Assembly line produced and showroom sales available"
In 1967 (3 of the 4) of the Stock Eliminator National Event Champions were older cars (Jr. Stockers)
Winternationals * 60 Pontiac G/SA
Springnationals * 58' Pontiac E/SA
Wolrd Finals * 56 Chevy Wagon
pc