I wish I could add much more but here are my (updated) notes on the 8x49:
[i]"This extremely rare cartridge is not shown in any known RWS catalog but two undated RWS factory drawings exist, both showing identical dimensions and numbered M244 & M245. The “M244” appears to be a mistake as M244 was already allocated to the 8x42R M88 (M4).
Examples of this case do exist with “R.W.S. N” and also “N 8x57J” hs. These are both likely to be factory produced from 8x57 cases but it appears never to have advanced past the experimental phase.
This may well have been an RWS factory experiment to produce a ‘middle length’ rimless M88 cartridge similar to the DWM 8x51 Mauser (M8). Factory drawing M245a (8x51) is dated 1923 so this is most likely to have been before that date. RWS factory drawing 243b (7x54 - W33) is dated 1922. Notice that all the similar M88 cartridges have consecutive numbers (8x42R, 8x49, 8x51) which may well imply a re-drawing sequence, or a later case drawing number allocation sequence.
The “R.W.S. N” hs would imply likely production prior to WW1 c1910 but possibly as late as 1920, so my best guess is introduction in the c1910 to 1922 period.
According to Peter Petrusic (via Peter Skala- RSACCA #195) this is a 8x49 Pre-WW.II German assault rifle experimental which was part of the 7.9x33 Kurz development but considering the factory drawing appears sporting, I consider this is unlikely.
The shoulder position appears the same distance from the base as the 8x51Mauser (M8) (see images) and it may well be that this would fit in weapons chambered for the 8x51.
All specimens known with “R.W.S. N” have an unusual sn jacketed bullet with flat top and a slightly stepped bullet (down to 7.8mm) (see images). [/i]"
Regarding the 8x51, Fede is correct about the "D.M. * K * " hs which could imply manufacture at least as early as 1896 but this hs is confirmed as being used much later c1902 or more. DWM case #366L is likely post 1898. The Roth case 681 is c1902 but DWM should have produced it first.
Incidently Fede, my RWS drawing #242 is the 6.5x57 M88 and has a later added writing with the date 1924 - do you have a drawing #242 with 6.5x54 case ?