I knew what you meant by floor sweepings, it's a commonly used phrase in my family. Dad wouldn't eat bread with floor sweepings in it, we won't eat bread without them.

Loads of frit was common early on. They were not able to import good quality cullet, it was often a greeny sort of colour - not good, as well as full of frit.
Perhaps they hadn't yet got theselves into the habit of sweeping the marver clean every time they passed it.
I've seen makers do that. It's automatic, just as twirling a rod becomes automatic when in a maker's hand. They don't know how NOT to do it.
But the fritty bits are not in the clear section of your piece, just the coloured bit.