I now have three Gilson test sieves, purchased over the last decade. The #20 arrived yesterday, so I made some careful extraction estimates.
We have an older Wolfgang Mock Grain Mill, that most closely resembles (and takes the same replacement stones as) the KoMo Fidibus 21. A comprehensive US source is Grain Mill, Flour Mill, Grinder | Stone Burr, Impact & Steel Burr Mills at Pleasant Hill Grain. They also recommend the Royal Lee Household Mill if one's exclusive goal is the finest possible flour. I would contact them for buying advice; the newer Mockmill 100 & 200 probably represent the best value and performance, if one is not put off by their appearance.
I adjusted my mill to grind as finely as possible without gnashing stones, then tested one and two passes of 500g hard red winter wheat from Central Milling, with each of my sieves. The extraction percentages are shown in the above chart.
One can buy Gilson sieves from sources such as Amazon, or directly from Gilson. It's worth consulting Gilson first, to understand options.
ASTM Test Sieves | USA Standard Test Sieve - Gilson Co.
What is a Sieve? Make Sure You Choose the Right One - Gilson Co.
12" Sieve, All Stainless, Intermediate Height, No.20 - Gilson Co.
12" Sieve, All Stainless, Intermediate Height, No.25 - Gilson Co.
12" Sieve, All Stainless, Intermediate Height, No.35 - Gilson Co.
These sieves are expensive but remarkably well-made. They fit perfectly into a Vollrath 69080 8-quart heavy-duty stainless steel mixing bowl, universally available from restaurant supply stores and Amazon. These bowls are the best quality I know; we have multiples of many sizes. I have a friend who cheaps out whenever he thinks it doesn't matter to free up an extra $40 for one night's wine, and his wife is still gnashing her teeth that he didn't get Vollrath bowls. The quality difference is striking.