Hey Eagleye.
Im the one who asked about your coin.
I target this type coin but almost never post in this forum for several reasons.
1. Chinese "cash coins" were in use in the U.S.(by Chinese immigrants) well into the 20th century. So, even though these coins may have been minted 200 or even 900 years ago, they are useless in dating a site in America.
2. Most Chinese cash coins were minted in the multi-millions and are not worth more than a buck or two. American detectors are aware of this.
In Japan however, sites can be more easily dated with "cash" coins as they were standard currency for a thousand years. More importantly for those of us who detect in Japan, there were bootleg counterfeit cash coins made in back barns and wood lots that are referred to as BITASEN (bad money).
While these coins were the scourge of their day, they were much in use, are always rare and often very rare and valuable these days.
There are official bronze Japanese coins that were minted too, some of these can fetch in the hundreds of thousands of dollars.
Also there are some true Chinese cash coins that are actually rare and attractive enough to fetch a couple of hundred US dollars and sometimes a thousand or so.
So for us detectors here in Japan, even a common chinese coin is useful in the tricky world of identifying all the cash coins we recover over here.
Your pic is a little tough to see though.
I use a lemon juice soak, maybe three minutes, rub down with baking soda, then rinse with distilled water. Dry completely then repeat once or twice if necessary. Gotta know when to quit though.
When you are finished and it is totally dry, rub it down with a little vasoline or baby oil then keep it out of the air.
That is how i have had the best success.
If you can get a clearer pic, I can help I.D. it for you.
Thanks for posting Eagle.
you can see it better in person, its too crusty in the picture for me to read well, so here is a chart to compare it to. 
