yes, plates are supposed to conform to a standard and pass its tests. Which
standards? With an item like this it wouldn't be worth your money without
it. Most everything in the mechanical trades are. These are useful,
interesting pages you could look up: (the information is endless). Knowing
the code after the standard name is a code itself, date of inception or mod.

AGMA
ANSI
ASA
ASME
BS
JAP
DIN
GGG
ISO
NEMA
SAE
ROCKWELL, Rc, Rb...
ABEC

just one standard by one org encompases all the codes for all the steels.
They team up. Buy copies. check their wwws.

Surface plates are sold by colour, The product should say what it is good
for. They get incredibly accurate. They are several feet thick and dozens
of feet wide and acuurate to a minimum of 1/10 000". Theres a million
analyses and surface finish symbols. You literally have to be able to put a
forklift on top of them and maintain this accuracy. Same with CNC machine
tools. Parts are loaded on to them with forklifts and machined to 1/10 000"
tolerances. You can get a dial caliper, inherent accuracy 1/1000" for $20
that is accurate to 1/1000". A dial is great. When you rock it you can see
the dial go cw or ccw, tell you the exact hi/low spot. A micrometer is
inherently accurate to 1/10 000" Screws are standardized for spiral runout,
and absolutely everything from the angle of the head to the length of the
unthreaded point, and the thickness of the plating, which is absolutely
check, in conjunction with the thread profile check: two checks, three
actually. Would ford buy unstandardized machine screws? Having markings is
a givaway. ASME/ASTM is an example. Good/high standards(several levels)
dictate what markings on the product are req'd to meet the standards. Size
printed on drill bits, with rules about minimum printing relief. At the
bureau of measurementds they actually keep a refernce of the size, say gage
blocks, got a million bucks?. In temp, humidity, dust, light, air, etc.
controlled . When you gotta check ya, you can base it on the speed of
light, but what do you do, set up two speed of light machines side by side
and do a comparison run? In some fields the entire science is driven by the
standards. People say they get in the way. You can't find your hand in
front of your face without them. All engineering in all tools are
calculated based on preferred sizes. Anything you can't buy is wasted
money. Machinerys Handbook, the bible.

sorry don't know the surface plate info.