Please help me verifying this 26 SKS

I got these photos from the seller. Can you guys tell me more about it? I only know it was made in 1964. I noticed that the rear sight has the D marking on it but according to http://www.yooperj.com/SKS-7.htm, it is from the sheet metal receiver version. Am I wrong?
The gun looks in very good condition from the photos.

Any help would be great.

https://www.milsurps.com/images/impo...09/NYwMP-1.jpg

https://www.milsurps.com/images/impo...09/aOtUH-1.jpg

https://www.milsurps.com/images/impo...09/IxN4j-1.jpg

https://www.milsurps.com/images/impo...09/QId1n-1.jpg

https://www.milsurps.com/images/impo...09/TewBs-1.jpg

https://www.milsurps.com/images/impo...09/0qiOf-1.jpg

https://www.milsurps.com/images/impo...09/gpatQ-1.jpg

Yoopers on SKS Page 1 has one in SN series starting 815 that he has ID'd as China 1964, and yours is 817 series. His SKS page 14 has receiver pics including China sheet metal, perhaps seller can compare to them.

I could be wrong, but in the photos, the stock looks like it might have been re-finished and stained a more "Russian" red color. Might just be the photos, though, as sometimes the actual color doesn't come through very well.

The "D" marking on the rear sight leaf is NOT limited to the sheet metal receiver versions. I have the "D" marking on a couple of my SKS56 rifles. These are photos from two different Chinese Police ("Public Security") rifles of mine:

One has the "3", the other a "D"...

https://www.milsurps.com/images/impo...sight016-1.jpg

https://www.milsurps.com/images/impo...ight0141-1.jpg

I bought the SKS. I found the gas tube on mine is really difficult to pop up. I need to use all my strength to pull it up. Is it normal?

Some are tight like this. I had an AK74 that was so tight that I had to use a plastic tipped hammer to move it. Once I got it opened, I simply bent the lever out (away from the sight block) to allow it to move more easily. A couple of very light taps with the same plastic tipped hammer is all that was needed.