Your Gew88 is a Mauser-Mannlicher hybrid designed by a German weapons commission shortly after the French came out with their own smokeless powder 8mm rifle.
Loewe was one of the private contractors to built the Gew88 rifle. They were in fact a company that supplied machinery and if I'm not mistaken this was their first foray into the arms business. They were major shareholders in the Mauser company, who was too busy making Turkish Mausers to take on the German contract for the Gew 88, so they built them at Loewe in the Berlin area instead.
Gew88 was a stop-gap rifle IMO. It was only 10 years later that the Gew98 was issued. What this means is that Germany built a lot of 88's and they went through a lot of changes over the years in terms of modifications and owners. (Turks mentioned) They saw front line service in WWI in some units, also many issued to reserve units.
There's the 88/Z (improved rifling - almost all of the early guns wore out their rifling very quickly) Your 1890 was certainly in this category and very, very likely has a replaced barrel. Look for a marking such as Z on the barrel and butt stock or...
88/S - rebarreled/rechambered for the Spitzer bullet introduced in 1903. Sights changed to reflect flatter trajectory of the pointed bullet. Look for an S marking too.
88/05 - same as S but modified for stripper clips
88/14 - same as S but modified (more crudely) for stripper clips
There are more variations.
Two important things: you need to SLUG THE BORE so you know whether it is designed to fire the .318 diameter round nose bullets it was originally designed for or the .323 diameter spitzer bullets that came out in 1903. You can blow up the rifle if you shoot large diameter bullets out of a small bore.
2nd: do not fire mil-surp or hot loaded 8x57 ammo in this rifle. It is not as strong as a Mauser action. The original S-Patrone bullet was only 154gr.
One of these is on my hit list for 2008
