Odd surface plate question

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So someone on craigslist is selling a couple of surface plates. They are from his father's estate and he says they are in good condition. So far so good. He has no documentation or knowledge of the manufacturer.
One is a basic 12x18x3 black granite, which seems normal enough. But the other is 12x18x0.75. I've never heard of a surface plate so thin and suspect that it is unlikely to be as stable as the 3" inch think version. But it will be much easier to move around the small area I have available. It also looks like it's two pieces bonded together horizontally. I wonder if someone was using it as a portable "good enough" plate. Even at that thickness it's probably a lot more accurate than anything I'm capable of.
I just wondered if anyone had ever run across such a thing. It comes in its own wooden box.
He's asking $45 for each one. Price isn't fabulous (assuming Grade B on the 3" version), but shipping is cheap since it's a 10 minute drive from home.


The thin one is countertop material; not to be counted on for any work with accuracy beyond measuring with a steel rule, not worth much. The 3" thick one is an OK deal, and likely quite good enough for most hobby shop work.
The thicker one is probably a Chinese model. If you can flip it over or tip it up on the side and see if the are 3 X's or 3 rubber pads. If it does it might be a good one but until you can test it who knows what you have. I also suspect the other one is a counter top. Depends on what your going to use it for. Suburban Tool on You Tube has a shows what's his name rough checking a surface plate. Go to YT and search suburban tool granite plate inspection and see how he does it. If you know someone at a machine shop see if you can get them tested on a CMM or track down a plate inspection service company.

The box is something his Dad made. Looks like it has roofing nails and a 2 x 2 side.
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