Hi Steve,
Welcome to the forum.
If I had to estimate the year of manufacture, I'd say around 1953 from the serial number.
To learn more about the fascinating history of the Mark 3, I strongly recommend you find a copy of Chris Thrale's book as it will answer any questions you may have on this classic air rifle. The book contains unrivalled information on the development and history of the Mark 3 that you will not find elsewhere.
As for restoration, I would clean and oil the rifle but no more. A refinished example is going to appeal to fewer collectors than one that has an honest patina from use. I think refinishing takes away a rifle's history too. Your model is almost 70 years old and a few knocks and dents are expected.
Enjoy it and welcome again.
John M
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.