Ok, I did a bit of "experimental archaeology" by 3d printing the shell based on the plans and then sticking the Md.83 double-effect fuze from the 63mm Md.83 Armstrong Mountain Gun on it and it's clear this was made specifically to fit this fuze.
When we contracted Hotchkiss for the 57mm guns for the Bucharest forts, we probably showed them a 63mm Armstrong Shrapnel and asked "can you make us this, with this fuze, but in 57mm?" which also explains why it's base ejecting instead of forwards ejecting.
And the reason they had it at the Paris Exhibition is because it was brand new, they hade made it for us just the year prior.
The only speculation that remains is about the lack of any written record about this shrapnel shell.
Now, the manual we have is from 1899, which is 11 years after first adoption and two things immediately jump out - first off, the gun is referenced exclusively as being "Model 1891" which is the time of adoption of the armoured turret in which it sits and it mentions that the only ammunition it shoots are the semi-armour-piercing base fused shell and the case shot.
Later documents mark this gun as "Md.88/91" for the field gun and "91/916" for the AA gun and the nose-fuzed common shell is referenced as "Md.1888" with the base-fuzed semi-armour-piercing shell being described as "Md.1891". From this I can deduce that, at one point between 1888 and 1891 both the nose fuzed common shell and the shrapnel were dropped from regular use, which is why the 1899 manual doesn't mention them at all.
in 1915, when the guns were taken off from the forts and made into infantry guns and AA guns, respectively, the M.88 common shell was bought back into use, but the shrapnel wasn't and instead was replaced with a more advanced one, made of steel and forwards ejecting.
So that's about it, I think. Here's more pictures of the shell replica on the real and a replica cartridge case as well as alongside the Md.88 nose-fuzed common shell.

