Evening EOD,
By the taking a first look at it, (especially at the cut away) It seem to me that you got a Projectile HEER (HE, Extended Range), as the wall of the proj. get thinner as it goes toward the nose. The shape is particular as well, as it's less bulky like normal proj. Instead, its more linear, more aerodynamic. And you got a space or cavity for a BBU (Base Bleed Unit) under the driving band. We do have in the Cdn Forces, a 105mm C132 HEER version of it. The cavity below the driving band is filled up with propergol cast composite propellant, which is a gas generator. The gas generated fills the wake at the base of the projectile thereby reducing the base drag and increasing the projectile range.
Note on the marking... I noted that all of your marking is stamped, not paint and near the driving band it is marked FES which should indicate iron rotating band (if my memory is good) which is marked on most of German proj. that I seen so far. And you mention that the diameter is 88mm... So I was thinking maybe German 8.8cm flak or artillery. I checked in what I have for German ref's and could not find anything related too or about it. Then again I'm kind of limited in ref's. So maybe this could put you on a new track... if you did not already went that way that is !?! lol...
Hope I have been in any help
Cheers.
FCAT.