Many people have told me the stock is not original, do you think the letter would tell me when/if it was replaced?

A Parker research letter will note if the gun was ordered with a Monte Carlo or high comb.

If a repair record exists for a stock replacement it will note that also. A stock replaced at Parker Brothers counts as original to the gun to collectors.

Did Parker make Monte Carlo stocks? Not that I would take the gun apart but would there be markings on it?

Yes. Such stocks were made. The head of this stock looks very original to me. the only odd thing is that the stock oval looks a bit larger than usual... but it could just be the photos/lensing.
A letter on this gun would be WELL worth the investment.

This is a very nice and interesting DH that you have here. Yes it has a few things that need addressing like screws and the pad.
The barrel finish is very nice. Looks like it still has a lot of good black / white contrast remaining. There are not a lot of good photos of the wood, so it is hard to say what it could benefit from in that arena, if anything. Maybe just some finish added and checkering cleaned up. It all depends on how nice you want it.

The pad should be replaced for sure.
The stock very well could be original. It certainly has age to it. If there are factory records on the gun, it would surely say if it was ordered with a Monte Carlo comb. If it was, then excellent! It makes a nice gun even nicer. Parker guns could be special ordered with this feature. You should order a letter for sure if there are records. That way the originality of the stock cannot be questioned.

By the way, the stock oval looks 100% correct to me. And it is retained with 6 nails as it should be.

The factory stamped serial number and the grade letter "3" will be stamped under the guard bow.

Brian, thanks for all the info. I will definatly get a letter. The gun does have the D3 on the water table. Does that mean it came with a Monte Carlo?

No, the D and 3 are the letter and number grade. The Monte Carlo stock looks very original and is a very rare option, indicating it was probably ordered by a competition shooter. The letter may identify that shooter. I will help you figure out who he was and what his claim to fame is. Don't throw the old pad away. A collector may want to have it to bring the gun back to original condition. Allow no modification to the butt that will prevent the old pad from fitting back on. Let us know about your letter.

The trigger guard has some initials on it...

"Monte Carlo" is the name that is given to this type of comb that is relatively flat in drop and then slopes down sharply just before the buttplate or pad.
It is commonly used by trap shooters where a higher comb is wanted, but a lower mounting point on the shoulder is desired. If these type of stocks did not have a Monte Carlo comb, they would end up looking like boat paddles.

The stock appears absolutely original and please heed Bill's advice.