Sequestered Ode to a 1903

Cut my Hours, cut my Pay
give me hours to fritter away.
Makes me think of my 03
and what it does mean to me.
There are some parts on my 03
I wonder what that they could be.
Cut in the receiver, rear mag well
is a slot where these parts dwell.
One is a flat spring, that's plain so see,
the other looks like a pin to me.
The pin goes in a little hole
and up into the receiver from below.
On the pin there is a tit
over the tit the spring does sit.
The pin is kept there by the spring
and it is such a little thing.
These parts are little, very small
and I assume, not strong at all.
On the diagram of the 03
these parts do not appear to me.
These parts, what is it that they do
this is what I ask of you.
What are these parts, what can they be,
JB could you answer me?

Regards,
John