60mm to 40mm Filter converters explained

60mm to 40mm Filter converters for respirators explained.
These were made for older 60mm NATO masks such as the M9A1, M61, British Light anti-gas respirators and the Canadian C3. The idea being a simple converter lets you put a 40mm filter on the 60mm mask.
These still work fine today and are very cheap meaning if you have for example a Finnish M61 mask and want to use it with modern 40mm NATO/RD40 filters, you can.