It´s an Eagle Defender.
General results:
1st and most important: the unid values of the items correspond with those of normal loot absolutely. This may not be directly clear as to why it important, but: that means the drops can be predicted by the range of unid values dropping from the chest. I did compare the unid values of all items, yet those usually have quite a huge range of unid values they can drop in - thus the general comparison might be an indicator yet not too strong evidence, even though just for the sake of completeness: everything fitted in. Differently it is on stats that happen only in one variation: inherent HSR staff mods on gold staves, ene for foci, shields armor and armor piece armor. second and last did not happen to me - I suspect they do not drop? I have far too few data (~50items for the istan island chests) to be able to tell anything about skins but it seems that armor pieces are impossible as well as foci (now that I summarize it like this this is interesting - if anyone could confirm it maybe? - I mean the foci part). So for those stats (HSR of gold staves and arm of shields) there is just a tiny range of unid values that enables one option: My overall data yields for gold/prpl shields: 15arm: 27-33g, 16arm: 33-58g and for gold staves HSR: 18%: 29-37g, 19%: 38-48g, 20%: 49-57g. You see there is almost a direct knowing from the unid value to the related stat. Now in my treasures runs I had many shields and gold staves (10 shields and 10 staves in istan to be precise) and compared the data - and it was all in agreement with my data from other sources. So we can predict the item levels, thus their possible DMG-Ranges and their possible reqs by their unid value range just as well for treasure chests as for other mobs. The only difference - which is no surprise - is the missing cutoff on higher levels. That is an extra feature which has been added to the drop generation: there are highest DMG on weps and highest unid values on unid items. So it seems that just the cutoff for the unid values has not been respected on treasure chests, yet ofc the max DMG cutoff has been ofc ::D
So the next point:
2nd: There are 5 categories of treasure chests:
istan (issnur,Mehtani) 31-49g unid items
kourna (Arkjok,Bahdok,Jahai) 44-62g unid items
vaabi (Mirror,Jahai,Ahdashim) 51-68g unid items
desolation (Ruptured,Sulfurous) 58-72g unid items
realm (Pain,Jahai) 66-72g unid items
in correspondance with the classification by region on wiki (I did not have all the min and max values from all the chests in a category yet, but I had similar and see huge discrepancies between chests from different regions, thus I´d say this one is correct) Also I do suspect the given ranges are not entirely exact, might get items a little out of range, since collecting data on these is slow... as said 50 items in istan so far (and corresponding in other areas).
Now the conclusion:
3rd:
a) istan type treasure chests will be perfect for q8s even tho it seems only shields and swords can be obtained of those weapons still dropping in max q8. They have a slight different unid value range than the lvl20 mob (whose range is 32-54g) but that is nothing to worry about if the entire theory is true since it obviously doesn´t have any lvl 19.75 or something - NO it just supports the point that items have a "level" and that the mob lvl is just used to automatically generate a rang of item "levels" to drop from them rather than typing in manually a table into the gw drop code that lists for all mobs what level of items they can drop - If questioning anything about the drops think of it like a programmer: how to you easily establish a drop algorithm that yields these results we see? so for treasure chests I don´t know how they determined the ranges, but I don´t have the limits clear enough yet as to make any conclusions.
b) while generating nonmax weapons which obviously are in consonance with the 44-62g unid range, kournan treasure chests won´t get you any q8s (except purple staves)
c) all the other treasure chests will be great to get max damage items but you´ll never in your life see a q8 there (unless maybe purple staves)
this is the closest we could get to "prove" something not dropping q8 - by figuring the drop mechanism and from there determine the limits
EDIT:
and you miss out: zaishen chest - no (I hope we don´t have to argue about that ;D - or you guys show me the q8 insc crysta, CC, BDS, Volti ...)
furthermore with these statements I think we could change the other treasure chests to a "no" (maybe even summarize it a little to make it easier to see - like treasure chests: istan: yes, Kourna: no, Vaabi: no ...)
EDITEDIT:
fun fact: also got an adamantine just like yours 
15-21 q8
