Are you sure they are true surface plates made to be FLAT, or are they only granite bases who's primary purpose was providing mass and stability to whatever was bolted to them........... Just saying................Bob
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.