I have had them with the mum on them and without the mum and it never made any difference to me.
This 7.7x58mm has the mum on it but the scope mount covers it up.
This one has the mum ground off of it and shoots just as good.
The last one I saw for sale at a gun show was selling for $200 and had the mum on it.
It also had a cracked mil stock and what looked like a 2x4 nailed onto the butt end of the stock for an extension :barf:
I would not have even paid more then $50 for a piece of junk like that mum or no mum.
You can go to this site to learn more about the markings on your rifle
If you reload for these Japanese rifles you will find that they are extremely accurate with 150-180 grain Sierra pro hunters and even with a 125 grain .311 caliber bullet when you use a little math to do it with.
My rifles can easily shoot three bullets thru the same hole at 100 yards with my reloads but if I loaded them with factory ammo it would probably increase the groups to a 1-inch.
The ammo factories use some math to set up I think then from then on out they depend on machines subject to human error run by people that failed their high school math courses LOL