I learned that a MS collector in the NE USA has a Mannlicher chambered for an 8x40 cartridge. Does anyone have information regarding an 8x40??? There is nothing about this caliber in municion.org, or any other source that I can find.

It is possible that the cartridge is actually the 7.92x40 CETME. Unfortunately, I do not know the model number nor the date for the rifle, whether it is post-WWI or post-WWII.