I received my recent shipment of Mann Lake PF-120 frames yesterday and as I was removing them from the boxes and placing them in freshly assembled and painted medium supers, I came across an interesting transition.
Check out this picture: http://parkerfarms.biz/Images/mlpf120.jpg
What I appeared to catch was the transition from one plastic supply to another. I even have some which have streaks of the older plastic while being made mostly of the newer.
This has happened once before to my knowledge. The original PF-120 frames (of which Michael Bush has thousands and you can see all over his website) were white. I have 20 of those originals. The frames that I have been purchasing over the past couple years have been yellow, slightly translucent, in other words you can see light through them if you hold them up to the sun.
However, these new ones are completely opaque, you cannot see light through them if you hold them up to the sun. They are of a slightly darker yellow plastic.
I don't really see any other changes. They've obviously not changed the mold, that would cost thousands of dollars, but it seems they have changed the plastic.