Replacing Main valve

[Nineplex]

My home was built in the 50's and am pretty sure that my main valve is original to the home. I have shut it off before and noticed that it does not shut the water off completely. Maybe someone could let me know if this particular valve could be replaced meaning the inner workings of it? I see that there is a fitting that looks like it could be unscrewed and was thinking maybe that part was replacable,of course I would need to have the service shut off at the street first.
Also was does the fitting on the side do?

[hj]

Even if that company were still in business, that valve's pattern would have been changed so many times that new parts would not fit. That is not an approved valve for your application, but it may just need a new rubber washer.

[cwhyu2]

best course of action would to replace it with a ball valve,

[Nineplex]

What does the fitting on the side actually do?

[hj]

If it is installed properly it drains the water out of the pipes when it is loosened or removed.

[Jay Mpls]

The valve is a stop and waste.The knob on the side is so you can drain it down a bit after you shut the water off.Plan on sutiing off or having shut off,the water at the street and adding this valve to your scap pile for recycling.It is most likely a gate type valve that has so much crud built up on the gate it will not seal.The other danger is the stems on these weaken over time.So you may get the water off and not get it back on.
Think about not having any water.I would rather sit in the dark!
So do your self a favor and if your skills are up,Shut off or have the city shut off the water in the street and replace BOTH valves.(You should have one before and after your water meter.)The one in your picture is a solder valve so I will guess that one is past the meter.The other one is likely a threaded type.
Good luck.