Looks like a block of granite with some threaded inserts put in it. Are there surface plate certification/calibration stickers on them? If not, I am afraid you are looking at tombstones, at least from a machinist standpoint.
I bought my 18x24x4 two ledge surface plate for $50, and had it calibrated by Standridge Granite. Sometimes inserts are put into surface plates for various uses, but make sure it is really a surface plate first. It costs a lot to get a tombstone made into a surface plate, it will have to go to the plant to be ground flat enough and then calibrated.