Decided to return the lens for refund regardless of if it is working perfectly or not. Will save up for some G class glass instead
Anyway, since I didn't use it much, I don't think it fair for me to do a full review but I will wrap up with some observations.
Above pic shows them end on. The older lens on the left is much bigger, taking 72mm filter size whereas the newer lens takes 62mm filters.
Side on fully retracted again the older lens is a bit bigger. Note the styling difference. The newer lens is similar in look and feel to other current Tamron lenses.
The size difference remains when fully extended to 300mm.
In practice both are similar in handling function. A good twist is needed to extend the through the range. The focus ring travels a very short distance so not good for manual focus. There is a zoom lock to prevent movement when carrying.
What doesn't show in images is the feel. The newer lens is lighter, and feels much cheaper. The more visible external parts seem to be constructed out of plastic rather than metal.
I would say if I didn't already have the old sample for comparison, I would have kept and be happy with the new lens considering its price. Only because I had the old one for comparison does it seem to come up a bit short. The reduced size and weight would help for general usage. Better bet might be to see if you can get the older one used off ebay. Note there was one model released in between the two here.