I believe the older cabinets were more consistent than newer models. I've owned three 5150 cabinets at some point or another. But for some reason they are extremely hard to sell due to the cabinets being blamed for the speaker neutrality that sound so different from other makes/models.
I also believe the newer ones are extremely overpriced - you can do better than a new 6505 cabinet these days. Other than that they are identical. 9 ply baltic birch construction with birch baffling and MDF backboard. No insulation.