Congratulations for your acquisition.
To answer some of your questions;
The "A" in the serial means you have an alloy trigger guard / floorplate. The alloy trigger guard / floorplate appeared in 1958 (more or less S/N 220 000), but there are known rifles (450 series) showing s/n as early as 209 ###A (end of 1957) with them.
After S/N 220 000 or so, the majority of the rifles will be found with an alloy trigger guard / floorplate and by the time all the steel one were gone, they used the "A" to differentiate them.
Regarding the bolt, you will usually find the three last digits of the serial number etched under the bolt lever. The bolt shroud and cocking piece have been stripped to bare metal, to look like a FN or a Parker-Hale.
Regarding the rear sight, your chances to find one original are slim, but you can try E-Bay and if you're very Lucky, the EE or Trade Ex...
It was pretty common on these days to swap the rear sights for Marbles model 97L, 66, 67, 68, 69H and/or 70H. The standard front sight height is .375" and the widht is 11/32" (Marbles front sight #37N).
The base you have may be a Parker-Hale (the width will then be 0.780") - if so, you need P-H rings, not common but they are there - but Redfield also made some pop-up peep bases and many others did so too. To find out, you will need to dig a bit in old catalogs.
If the width is 0.825"-0.830", then, you should be good to go with standard Weaver type rings (the Grand Slam would be a good choice for me). The base with pop up peep is a very good feature and they are not too common neither.
The original buttplate was made of black plastic. They're not very easy to come by and repros are not very well done, and anyways it's pretty thin and may well not fit so good after being installed and sanded to fit the stock contour, plus the LOP will be pretty short if you remove the pad.
Now, a lot of these rifles were modified before they even leave the store, as it was pretty common in those days to have a lot of aftermarket stuff fitted on the gun. Take it like a new girlfriend; she had previous life before you and that's part of her heritage. A lot of these aftermarket accessories are now long gone from the market and in general, they were much better made than those of today - no plastic was used!
Anyways, the rifle will gain very little, if any, value by trying to get it "original" (as "out-of-the-box").
My conclusions are; the buttpad seems to be fitted like it should and if not dried out, I would leave it alone. You still can have the bolt shroud and cocking pice blued, but it's not 100% certain the blue will be the same, unless your gunsmith can garanty you it will be... As for the rear sight and the existing base + peep, if you can find the right rings to fit the bases and the peep can be adjusted to shoot right (which I don't doubt), there is little need to install another rear sight. And you still can look around to find one while using the rifle.
If Weaver rail type rings can be used, you will need to get a vintage scope to fit the rifle and enjoy.