looks like powder coated "something". AR500 plates (AR500 being a heavy wear, hardened steel) are common for things like impact plates on digging equipment or air chisels to break up concrete, and are also common in steel "gong" type targets. they are indeed level III rated if thick enough (from 3/8" through 5/8").
however if shot from too close, it can shatter or just have a hole punched through it. AR500 steel also has to be treated properly during the manufacturing process or a bullet can riccochet and deflect in a bad direction, or if it's too soft will chip away rapidly after only a couple strikes.
additionally when a bullet strikes an armor plate, the bullet shatters, sending shards of copper/brass/steel outwards at high speed, cutting through most materials. the lead is soft enough to splatter. the razor-sharp, shattered jacket/core material is "spall", and a spall liner is meant to catch these fragments before they reach flesh, however it's not typically required if the plate carrier has an external liner or the wearer doesn't care.
let's assume the plate in the photo is a level III plate, adequate to fill the CO CPC, it would weigh about 8lbs by itself. that doesn't look like it has a spall liner, so i'm going to assume it's just painted or epoxy coated or whatever, and not spall rated. such a plate is about $50-60 depending on where you buy one, and generally you want to wear two of them (front and back).
pictured below are a pair of my 10" x 11 3/4" level IV rated kevlar/ceramic plates from 3M's MaxPro line (without the cordura spall liner). these are 7.9 lbs each, so these are a similar weight to the level III AR500 plates. the ammunition in the belt is some 7.62 NATO armor piercing i had laying around for size comparison and to illustrate what it can reliably stop.
notice the kevlar spall liner glued to the back of the ceramic (yellow color fiber sheets on the right side). the ceramic plate itself has a thin kevlar weave glued to it as well (left side).
what's weird about the pictured plate is the texture (which is why i assume it's just painted and lacks any spall protection - the CPC itself isn't spall rated since it isn't kevlar or 1000D cordura, but 500D instead. not terrible, but not rated for spall). the shape at the corners is also a little weird, as plates tend to have shoulder cuts. SAPI and ESAPI are sometimes smooth like that, but are back plates/side plates. not chest plates.
all that being said, it's kinda dumb to wear it under clothing, as attachment webbing is meant to be used for attachments. concealment armor is generally soft armor, level IIA or some high speed IIIA stuff like Galls.