Any ammo experts out there?

This is a rare instance...it is one of only a handful of calibers that were used by both the axis and the allies in WWII.  the British used them in the Besa machine gun, which was most often mounted on armored vehicles in place of the Vickers machine gun.  It was a very reliable weapon....but anyways, that probably isnt where this one came from.  

You surmised correctly that the "S" stamp is for Spandau, the manufacturer.  The "10" means that the round was part of Lot #10, and the "17" means it was indeed made in 1917.  The S67 means two things--the S is for Spitzer bullet, while the 67 shows the alloy mix of the cartridge case--in this case, 67% copper.  I would have to agree that this round was a machine-gun capable round made in 1917.  It is not the S-67J, which would normally predate 1905.  It also isnt the I, which would normally be used in infantry rifles.  There was also another specific round, known as sS, which was issued to snipers.  The bullet itself was a very different design when compared to the standard Spitzer bullet.  Hope this helps

Thanks for the response F-8!

I've had a hard time tracking down what info I have so far, so it good to see I'm headed in the right direction. 67% copper sound spot on as the bullet is quite strongly copper colored in comparison to many pictures I've seen (the bullet also looks to be a standard Spitzer, nothing special).

I had initially thought that the "segments" were part of the original headstamp, but after a bit of searching it seems that quite a large amount of ammo was reworked with an annealed (or re-annealed) case neck to make it more suitable for machine gun use. Story goes that after the rework the already headstamped cases were additionally stamped with a large X to identify them, giving the appearance of the segments - does this ring true?

Thanks Again,

Martin.