I would think the Jnr denotes the package. Basically its a double hum bucker in a single coil form factor so you don't need to modify the scratch plate.
Is your JB a double?
All my Strat pickups are doubles in a single coil packaging. I'm a little disappointed that I cant split the Duckbucker, its staggered, but the little 59 on the neck is smoking hot too.
The most impressive I've used are the Lindy Fralins on my Tele. I bought them in NY on recommendation 2 years ago. I think they were called the Blues specials. They have a 5% overwind and bring out rich warm tones. seem to take the edge of the twang a bit.
Take it easy . Good making contact.