Yes I remember reading that now in my big book of turn bolt rifles. It keeps gas From running down the track for the bolt lug and into your face in the event of a ruptured case or broken bolt or some catastrophic failure
The Pre-64 model 70's all used the same overall length receiver. The bolts for the long and H&H length cartridges didn't have a piece like that, but the bolts used for the short cartridges; .243, .308, .358, etc. did have a piece similar to that. It was brazed onto the extractor retaining band and acted as a stop so that you didn't have to retract the bolt all the way back when cycling the action. The photo shows a .300 H&H, .270 Win and .308 bolts.
I have 29 Mod 70s and never saw such then again I don't have a new production one made in South Carolina and now that I have seen this I will not. I live about 50 miles from where they are now made.
That means you have a late production striker (firing pin) the late model ones are notorious for breaking the cocking cam off the back of the striker. I broke three of them in one year ! ! ! ! I was doing lots of competition in that time frame. You will be able to wiggle the bolt but it won't raise and only way to get it out is pull the bolt out of the rifle and unscrew it.
I would get a couple of them for spares. Numrich arms probably has them. Also if by chance you have access to a pre 64 striker assembly it will interchange with the new one.