It's nice to know that the Tefba filters are still available...
My cousin, who owned the business, died a few years ago and her husband, who ran it and invented the filters anyway, now has severe dementia. They are definitely good.
Now, a couple of things to consider:
1. To get more rust out, stand the block on end, block the holes and put in a mixture of 1 part of molasses and ten of water, leave for a day or two. That kills the rust and should free up more flakes.
2. Do NOT use brass plugs. The plugs will live, but the galvanic action will damage aluminium parts like the water jacket in your manifold, your water pump and your timing chest. Of course, it shouldn't if your coolant is good, but it's still an issue. As an example, Peugeot 604 and Volvo 246 share the same block, Volvo use a brass drain plug, Peugeot don't Typically all Volvo blocks, if they haven't corroded through, are like mush inside. And that would be with coolant used.
Have a look at the galvanic scale.