China M98 is part of the importers mark. Most importers call any Mauser 98 action an M98. They don't understand nor care about specific model designations.
For old grease, cosmolene, and tar/crude on stocks and metal I usually use mineral spirits. Test in a a small discrete area of the stock first to make sure it won't affect the finish under the crude. I've never had it to but it's a good idea to err on the side of caution.
I can't help you with your other questions but I'm sure there are others who can. Especially on the significance of your serial prefix and the locking screw thread size. If you don't receive any answers here I would post your questions on the Mauser Forum on these boards. I've found that a lot of the Mauser knowledgeable people don't check the Czech weapons boards very much.
"A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing."
George Bernard Shaw