Craig, that is very similar to the 9.3x57R Mauser M88 (see below) which you first mentioned. It uses c1.0mm larger case diameter than the 9.3mm Express cartridges. Why the “E” title was used could only be explained as a mistake or the the chamber was modified to this M88 case type which I would think is possible ?? But why - because it would be even harder to find ammunition for the rimmed 9.3x57 Mauser !?!
Here is information on that caliber
********************* 9.3x57R Mauser M88 (M67)
This virtually unknown cartridge is a rimmed version of the 9.3x57 Mauser (M40). Collectors may possibly have examples of these but have mistaken them for the 9x57R Mauser (M37).
A DWM factory drawing exists that shows that DWM case #491C is a 9.3x57R based on the M88 Mauser case, effectively a rimmed version of the 9.3x57 Mauser (#491 M40). The date on this drawing is not clear but is believed to be 6 June 1906. An actual case is shown in the DWM 1908 supplement to the 1904 catalog - see images. A DWM example is known to exist in the Heinz Held collection and it does have the expected “DWM K 491C K” hs, similar to others of the series but it is believed that no full DWM commercial production has occurred for this cartridge. Will Reuter believes that “fakes” have been produced with the “DWM K 491C. K” hs which is what you would expect such DWM production to have - see headstamp images. Such a genuine cartridge would be very rare and worth hundreds of dollars.
The only RWS catalogs that this was listed in were from 1924-1932 (“as stocks last”) catalogs. An RWS factory drawing is known dated 12.11.1929 (possibly February 12, 1924.) that shows this case type with an “RWS 9,3x57R” hs but only “H.UTENDOERFFER NÜRNBERG” hs examples exist. G.C.Dornheim (GECADO) also listed a “9.3x57R” which is most likely this case type (rather than the Express) but these were likely RWS production. RWS production may well have been quite a lot later than DWM despite the “Utendoerffer” hs. It was also shown in the c1926 Steigleder catalog (page 61) as “9,3x57 ohne und mit rand” similar to the RWS listings which probably indicates that this was RWS production.
There are slight differences between the RWS and DWM drawings, mainly at the shoulder (DWM = 10.65mm RWS = 10.95mm) but this wasn’t unusual back around the beginning of the 20th century. Also the rim thickness seems to vary from c1.2mm for the DWM version to c1.4mm for the RWS type. However, it does appear they were intended to be the same cartridge - a rimmed version of the 9.3x57 (M40).
Note that the DWM 491C hs appears to be for a rimless case (ie it is smaller and closer to the centre) but is actually on a rimmed case. Which leads us to the following comment:
In late June 2008 at Sörmland, Sweden, two guys (Anders Lindell Sörmland and ? Sparreholm ?) thought that they had ‘invented’ this ‘new’ casetype by combining the 9,3x57 and 8x57JRS but they were over 100 years too late. They called it the 9,3x57R.SE Sparreholm Express (see Headstamp images and www//93x57r.se/index.html).
Reputable source “Fede” (IAAForum) states: “For what is worth, this caliber was listed for years by Quality Cartridge although I don’t know if any cases were actually produced.” This has yet to be confirmed by me.