I saw your post on Gunboards-98k, probably should have done it on Gunboards-Mauser, as this subject needs a specialist and you will not find one on Gunboards-98k. You might get some intelligent comments on Gunboards-Mauser though, TP's forum is still excellent for German military rifles.
As you state, without pictures or a detailed description there is only a limited amount of things one can say about your rifle. You mention bluing, it is impossible to judge whether the bluing is original without pictures, even with pictures it is problematic. A faded or weak "B" on the siderail isn't much to go on, the acceptance stamps and serial/fireproof would be at least as susceptible to any wear associated with re-bluing. Typically the acceptance show more fading than a siderail, though a weakly struck siderail is also possible.
An original 98b stock would have grips and a takedown. If it lacks them the stock is almost assured not original. There are some RM (naval) reworks that have G98 type stocks on them, meaning no side sling arrangements, but these are in a narrow range, and from recollection they have grips and takedown.
The "S/42G", I assume you mean the rearsight sleeve, the mount that wraps around the barrel and forms the foundation for the rearsight assembly, - the nomenclature varies by collector, generally, but usually this is called a RS sleeve, however, just the fact that any part of your RS is S/42G marked means it has been at a minimum replaced, probably a subsequent upgrade, which most 98b saw to one degree or another. Anyway, the presence of a S/42 RS part has no influence of whether the barrel is replaced, they are two different things (parts) and often RS parts are replaced. The markings on the barrel will determine whether its original or replaced, usually found under the stock.
Little can be said beyond this, you do not even give information to determine a range for the rifle, and Simson 98b production is broken down by variation (
there is no formal chart, my break down differs from the one Mike used, but they generally follow a pattern based upon marking pattern changes), a type II (where the Karab.98b designation was introduced, a late a-block) through type 4 would all have the same siderail designation. Only the last block changed the SR...
The only clue given that gives some indication of a problem is the stock, you do not mention length, and a 98b should be the same length as a G98, and the stock sounds like it could be a 98k stock. Which is 6 inches shorter. This is not neccessarily a problem, it could be a conversion (not likely as they are rare) or they could be recycled 98b receiver built as a 98k, which is far more common. Or it could be a postwar rework or sporter rescue, or a great many things as well.
When you get it do some pictures and we should be able to help a great deal more. I would not waste time with Gunboards-98k, but posting about your rifle on Gunboards-Mauser might get a few knowledgeable comments.
Good evening all. I have a shot at picking up a k98b. I haven't seen it in person yet, only photos. Unfortunately I didn't take them and can't post them here. Anyway here it goes. It has matching reciever, barrel and floorplate. No details about the bolt. It may or may not be reblued as the B in karab 98b is hard to see in the photos. The stock appears to be a standard k98 without grasping grooves. The thing that threw me was the rear sight base was marked S42/G. How common would that be to have the S42/G marking with the correctly numbered barrel?
Thanks in advance, I know pictures would help but not possible right now.
JD