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Stand too? I hope you don't get arrested for grand theft...
I have two cast iron plates, neither as large as yours. I recently got a 14 x 18" Brown & Sharpe surface plate for $60, but it needs to be scraped in again after sitting around for decades. At least it was protected from rust. Has a A.E.C. tag on it (Atomic Energy Commission.) Does not glow in the dark.
It looks like it might be a Challenge brand surface plate. I purchased one of the same size with a similar stand from a shop that was downsizing last year for a similar price. They are still made and are quite expensive new. If I remember correctly a machine ground plate of that size sells new for a little over $3,750.00. A hand scraped one goes for a little over $4,200.00
If you do an advanced search on eBay you'll see there is a huge difference between what they sell for new and used. When I was looking I found some people asking near list price for a used stand and plate. When I went to the advanced search and included sold items I found they often go for around the price you paid. When surface plates get to this size they appeal to a limited audience. Most are sold locally since the cost of shipping is prohibitive. The plate weighs 475 lbs., and the stand another 100 lbs. or so. The shipping would in many cases be more costly than the merchandise.
Good question, Emilio. I have never seen the answer or any discussion on the question. Regardless, they are generally used for different purposes, depending, so the question may be moot. Both move with temperature and with loading, and cast iron often moves with time. Granite surface plates wear less and are much quicker and easier to calibrate than cast iron plates, unless the iron ones are ground. Smaller cast iron plates are often used for machine repair and calibration by taking the CI plate to the machine. Granite plates usually have the work taken to them. The advantage of CI plates is that they are much easier to calibrate yourself, if you have the scraping skills and an accurate master plate, usually granite. CI plates are also used for less than high precision work in shops not in need of it, for layout and less critical measuring and testing work. The CI plate can be used on rougher work, and then re-calibrated as needed in the shop, without sending it out, but again also requiring a good master plate. As purchased, ground CI plates are typically less accurate than scraped ones, and require regrinding to return them to original calibration. More than you asked for, and no real answer, but perhaps that helps...
As a side note most shops that use surface plates for precision measuring are climate and temperature controlled. By in large the temperature is controlled at 68*F + or - 4* humidity is usually controlled between 35% and 45%. At least these were the parameters where I worked. They are also the parameters I use for my shop.
$50.00 is an excellent price. For that kind of money, if I was at all interested I would gladly drive the 3.5 hrs. Look at it as more of a road trip to shake off the winter cobwebs and see the Midwest in all its spring colors.
Personally I have all the surface plates I need. I have one 12"x18" and one 24"x36". Having said that I'll be driving a couple hours this afternoon to check out a jig bore at an equipment dealer. Some things come up for sale at inconvenient times and in inconvenient places, but if it's what you want, and the price is right, the drive is worth the effort.