1915 dated Kar 98 from the German state arsenal at Erfurt. The EWB brand on the stock relates to the Bavarian post WW1 militaria group that was found in Bavarian after WW1 primarily to stop and prevent, I think, communist spread into Germany (or Bavaria for that matter). Remember, less than a year before, Imperial Russia had just fallen to communism so it seemed to be a very real threat in post war Germany.
That tells you where that carbine was in 1918, in the hands of a Bavarian soldier who was part of the EWB (can't remember the exact name off the top of my head).
The "7.91" is the land diameter of the bore as measured at Erfurt in 1915.
The "8x57 IS" looks to be a post WW1 applied marking and I believe it to be U.S. in origin though I may be wrong. It is the first time I have seen this marking.
The eagle on the forward left of the receiver is the Prussian property mark denoting acceptance of the firearm into German military service. The original serial number is 5286 L (?).
I believe the side ways eagle underneath is Weimar republic era in origin. I believe that eagle marking combined with the "572" denotes possible police use (seems to have been a common post war use for these carbines). The "572" and antlers may also relate to the EWB but so few of these carbines and rifles are documented so it is difficult to say exactly what is what with marked examples.
The "WAESCH SCHW HALL" marking, I haven't a clue. It may relate to the EWB posting or perhaps possible police use.
You do have one nice walnut stock, clearly marked for German ownership (originally Prussian by the looks of it) that ended up with a Bavarian unit at the end of WW1. The stock is a "Type I" that classifies it as a walnut type for the wood, without finger grasping grooves, and without take down disc. It is the first pattern that the "lengthened carbine" was adopted with when accepted for service in 1908.
It looks like your bayonet lug is missing, do you have a picture of this area in detail, I am curious to see what happened here.
Overall, a VERY nice carbine with plenty of history.
As far as value, matching numbers on Imperial German rifles/carbines can seriously affect the price in the market it seems. Some people may not like the "EWB" brand while others will appreciate the historical value. From the few Kar 98s in full military I have seen, the price ranges between $600 up to $1000 depending on condition, maker, unit markings, date of make, battle damage, matching numbers...etc.
http://en.wikipedia.org/wiki/Schwäbisch_Hall
SCHW HALL is apparently a place . A little rusty here,

But I think Schwaben refers to a district in southern Germany. Who knows, this piece might have been with, or against Herr Hitler, at the Beer Hall putsch.