[03man]

Looks like the reason you can't get the lock out is that they made the firing pin diameter larger, compared with the other one; as it is the same diameter of the spring seat, instead of "stepping" down in diameter.

Probably why there is the center groove in the lower piece that latches and unlatches the lock.

If it works when together it is likely not a problem, you can check the movement of the lock out of the action.

 

[03man]

You can't see the "step" in the firing pin diameter behind the spring?

Were the shotguns "working" when you took them apart?

I don't have an 870 that old, or I would take it apart and see what it looks like- but by 1980, approx. date of mine, it may have changed again.

 

[03man]

I think you are trying to fix something that ain't broke, JMHO.