This is the third 1917 LP08 reported marked to the 57 JR. Another is 1917 LP08, sn 3066e, J.R.57.6.K.P.6. This Luger was imported from Canada by Simpson some time ago.
The below LP08 is from the same regiment and is stamped the same other side way.
* 1917 LP08, MACHINE GUN MARKED Jan C Still (1917 LP08, sn 7670f, J.R.57.1.M.G.K P.7)
http://www.gunboards.com/luger/topic.asp?TOPIC_ID=631
I would suggest that these three Lugers marked to the 57JR are all correct considering the unusual method of indicating the pistol number and the various source locations and long reported history.
Jan