Definitely the back. The original source showed both the soft and hard inserts still inside of their sealed plastic bags. Discolouration is probably from dust/residue/wrinkles that accumulated over its years of storage.Although 1) is this the back of the plate or the front? Based on the printing my guess is back. But the discolored spots could mean this was the strike face and those are fragments of the round
2) why rate their plates to stop ammo unique to them? It reminds me of Zimmerit treatment on mid to late WWII German tanks. Okay fine it will stop magnetic mines but only Germany issued Magnetic mines.
3) is this supposed to show the strength of the plate or weakness of the ammo? And does it state beyond 5.8mm what type hardened penetrator core tungsten or softer training ammo?
Likely DBP-87/95, as per GJB 4300 standards.As for 5.8, well the Chinese 5.8 has superior armour penetration capabilities compared to 5.56 NATO and Russian/NATO 7.62, so if it can stop 5.8, it can also stop all standard rounds from other major militaries.
I would guess that the plate was designed specifically to stop 5.56 and 7.62, but would need additional support from soft Kevlar to stop 5.8.
Nothing wrong with that, it seems perfectly adequate for simulating the generic small caliber high velocity rifle round. If it stops DBP-87/95, it's probably going to stop 7N6M or M855/A1.
Now what I'm more interested in is the weight and material composition of the plate.