My jelly bag must be something along these lines. It's used to strain all the fibrous material out of boiled fruit to clear the juice to make jelly. That's jelly jam, not table jelly.
The cloth is simply very tightly woven wool that over time and use becomes almost felted. It seems to last forever, mine's over thirty years old and it gets used every year. The cleared juices slowly seep through and drip off the bottom. They end up crystal clear.
Newer ones are made of nylon and they're dirt cheap.
http://thecookskitchen.com/browse_2549
Mine looks more like this cotton one,
http://thecookskitchen.com/browse_2547
I used well washed fleece fabric to strain elderflower juice last year when working with kids, and it was really successful, no clogging even with all the fine pollen and tiny petals.
Any idea what the Millbank bag is treated with? 'cos otherwise it's a dear bit of cloth.
Cheers,
Toddy