Tannoy 12" Reds vs Golds

I am trying to understand if there is a difference between the 12" drivers in Tannoy Little/Super Red Monitors and Super Gold Monitors. Both 12"drives are 3149, but i believe the golds are 3149b. Don't know what "b" means. I have the SGM 12X, and have always been thinking that they are inferior to the Red series. Now, it turned out that the drivers are the same, so my concerns may not make sense.

Do you know what the difference between the Reds and Golds really is?