I assume the confusion is about the cobble-generator, not the lead+aluminum-dust = silver.
It's a water-lava cobblegenerator using split streams, so 1 source block of each type divides into 2 flows, effectively turning it into 2 generators at once, which all come together right in front of you, so you've got 2 faces of cobble at 90' to each other...And really the only limit is 'how fast is your mining pick?' Takes about a minute to put out a stack using a bone TiCon pick. I kicked out a few stacks of cobble before this just to put together a nice oreberry bush cellar and turn the top of the Fry Queen into a cobble-clad spawner, then a couple more just to see what I could get by running one stack each of gravel, sand and dust through the sieve (quite a lot of certus quartz).
Couple of screencaps in case you haven't seen the design before. I had to spam F2 to manage to get a shot with both faces of the lava exposed.