What would such a stack display as? "Mixed bolts"
Stack Id and Brem's plump helmets together and that's great, but there's no reason for plump helmets to stack with dimple cups.
I'd like to see -steel bolts- stack up, whether they're made by Urist or Cog, but I don't even want Urist's -steel bolts- to stack with Urist's +steel bolts+. This isn't exactly the same, but many of the same problems arise.
Stacked objects are supposed to be, for practical purposes, interchangeable. Nobody but Urist cares that he made those bolts, but Urist; we can't even tell he made them. Which bolts are actually being fired needs to be tracked, so we know who freaks out when his masterworks get broken, but it makes sense that we as the player don't know who made which ones(a fact that is only important if one of the dwarves in question is to dead to worry about his masterworks.). However, the quality and material of those bolts have distinct, concrete effects that need to be kept separate.
Put em in the same bin? Fine. Store them in the same quiver? Fine. Mix them in the same stack object? Why!?
Stack the bronze arrows next to the iron ones, or even in the same tile, but you don't need to merge the stacks. There should probably be an (O)ption for "Dwarves re-stack damaged arrows," since whether having inferior stuff mixed with new is worth it, is a matter of whether you have enough new stuff at the moment.
Why try to make a system that can somehow communicate to the player all the several types of items in a single stack, instead of making a stack for each type of item. That's more work for Toady and doesn't give us anything really useful back, when they could just be two sacks of similar bolts.
Moreover, how big should arrow stacks be allowed to get? You don't want one dwarf cramming every arrow in he fortress into his quiver.
Being able to reach inside artificial stacks and manually Melt/forbid individual true stacks (which would then fall out of the stack object) could be useful, if you late want to , for example, remove a damaged arrow that was re-stacked.
One point I'm not sure about is bone bolts of the same quality, but from different animals. should -kobold bone bolts- stack with -horse bone bolts-?