It wasn't mentioned above,but your rifle was proofed in Germany,not in Belgium.Based on this,and your description, your rifle was likely to have been built in Germany,as well.Rifles like this are often made on M88 actions, and by 1933,surplus M98 actions were readily avaliable.How sure are you that yours is an 89 Belgian action. American made 8mm Mauser ammo(Rem.,etc.)is perfectly useable in your rifle.
The action does not have the Mauser full length extractor on the right side of the bolt.
The Belgian 89, Argentine 91 and Turkish 1890 are 3 Mauser variants that use the small short spring loaded 'claw' set flush with the bolt at the head.
An advantage of that style is that you can very easily close the bolt on a rd dropped into the chamber. But controlled feeding is not a sure thing as with the standard MAuser extractor.
The 1889 Belgian and the 1891 Argentine are the same rifle for the sake of conversation, the Belgian variant uses a sheet metal heat shield around the bbl. They both use a 5rd straight line feed magazine, the magazine extending below the stock line in front of the trigger guard.
The Belgian 1889 was mfg by FN and a very few by Hopkins & Allen in Ct/USA. The Argentine 91 made in Germany and in Argentina under license. The Turk was German made.
Another feature I note is the long sweeping contoured arm off of the bolt release that fits the contour of the rear ring on the left side.
I don't recall any of these using that feature, though they may have.
The later Turk Model 1903 used it. But that rifle is a clone of the Spanish 1893 Model and used a flush mounted 5rd staggered mag and standard Mauser claw extractor.
Someone had to have converted the 89/91 type action from it's single stack feed system to the 5rd staggered flush magazine w/floor plate
The 89, 91 and even the Turk 1890 rifles were all orig in 7.65x53 Mauser cart.
That got to be known as the 'Argentine MAuser rd' when those rifles were all over the surplus market.
Early reloading dies and some ammo was marked 7.65 Belgian & Argentine Mauser however. Some people still recall it by that name.
re: .318 dia 8mm bullets. I've used .32 Winchester Special bullets in my reloads for the round. They measure at .320 usually and are very thin jacketed. I've never had any issues using them but I always use starting loads to kill paper targets.
I use standard 8mm loading dies, but I don't use the Expander button.I just resize them and seat the bullet in the case at that sized dia. The small(er) dia neck seems to work out fine.
You have to de-prime separately of course but it's not a big deal.
I shoot them in M88 sporters.