These undersized expanders are for 6mm.
These fit into either of the STD or Tall 21st Century Expander Die Body (purchased separately). https://www.brtshooterssupply.com.au/product/21st-century-expander-die-body/
They are in .005″ (half thousandths on an inch) increments and stamped so on each expander.
For example: The engraved sizes are 0.2400″, 0.2405″, 0.2410″, 0.2415″, 0.2420″, 0.2425″, 0.2430″, 0.2435″, 0.2440″, 0.2445″, 0.2450″
Please note: Like most machined products there are tolerances. What we measure, with good equipment (micrometers), is in the range of up to 0.0002″ (two ten thousandths of an inch) either side of the stamped size. For many purposes this would be acceptable but for others, maybe not. Therefore, we are pointing this out before you purchase!
Custom expanders are deemed by manufacturers as non returnable items