I hate those Jensens that came in Twins. Junk IMO. The greyback JBL D-120's were available starting in 62 or so (I think) in the "Twin Amp" but I don't believe it became a factory option until the late 60's in the drip edge models. I'm no historian but occasionally you will see a Twin with a small JBL badge on the lower right hand of the grill cloth. That is a factory loaded model. I want to say the upgrade was $300 or so. Serious money in those days.
There is so much alnico in them they would probably run 600 or 700 each if they were still made. Every other alnico speaker is basically a cheaply made piece of shit by comparison. 4 inch voice coils, edgewound. They are going for crazy money even blown right now. Probably not the best choice for what you are doing with the seven string but for RnB they are just a great sounding speaker. Duane Allman, BB King, Freddie King, many greats love the JBL's. Here's BB on a factory loaded model, notice the badge.