David,
This mark doesn't designate any nominal "caliber", rather it is the bore( not groove or bullet) diameter, expresses in gauge measurement. It is often found on various nominal 9.3 or 9 mm caliber rifles, mostly rimmed hunting calibers like 9.3x72R or 9x58 1/2R S&S. In my experience barrels with this mark "slug" to about .358-.359". Unless there is some other indication, a chamber cast will be required to determine the necessary cartridge reliably.
Mike