Would Collath have had gunsmiths working in house for him at this time? You bet!
The company was founded by Wilhelm Collath's father in law Gustav Teschner in 1838, taken over by W. Collath about 1870. When Wilhelm died in 1906, his sons Franz and Paul Collath continued. The company existed until 1945. Various names were used over the time: Teschner & Co., W.Collath & Söhne, Teschner & Collath, trademark TESCO. Pre-WW1 the Collath factory was one of the largest sporting gun and ammo maker in Germany with more than 100 employees. Granted, most of them may have been occupied in cartridge and bullet making, but Collath certainly had full gunmaking capacities. They even supplied many country gunmakers with parts and complete guns in the white. Remember, government proofhouses were few: Two at the competing neighboring Gunmaking centers Suhl and Mehlis, one in Oberndorf, site of the Mauser factory, and one in Frankfurt(Oder) at the Collath factory. The former Collath factory was located at Krossener Str.18 in Frankfurt's suburb Dammvorstadt on the east bank of the Oder river. As the Oder since 1945 is the border between Germany and Poland, so it is now in Polish Slubice.
As to your question about the barrel diameters, please have a look at this discussion (i hate to repeat myself):

http://www.germanguns.com/upload/showthread.php?584-8x57R-and-8x57JRS-question

The barrel dimensions you give,.309-.323, are the ones of the post-1891 military barrels. But the real critical dimensions here are those of the chamber's case-neck and throat area. At least, your rifle was not deemed fit to use the WW1 S load by the authorities then, evidenced by the holes for fastening the warning plate on the stock. This is what I would do: I would take one of the downloaded American 8mm Mauser loads. If it chambers without the slightest resistance, shoot it. Then take the fired case. If a 8mmS = .323" bullet slips freely into the unsized neck, you may use the rifle with all modern 8x57IS factory loads.