Nice find...I just use a Korean era Garand ammo belt. They make repros of Garand ammo belts that are a few bucks more than the buttstock pouches. Sometimes they can be found on eBay for around $35.
I've seen the belts before, but hadn't seen the butt stock pouch. I like having ammo on the gun, but some people don't. Makes it harder to forget ammo when deer hunting though. Don't ask me how I know.....