Looks like a casting defect. And an odd one at that. I would like to hear what some of the other members who collect these have to say. Until I find out more, I'm begrudgingly leaning towards authentic. Detecto - the photo works. Next time, try placing it on a plain white sheet of paper. The idea is to get a uniform background. Please measure it precisely and weigh it. Where did you acquire it?
coin was purchased from a guy who got it at an estate auction. Each coin in a bag with a paper which had the info written on it. All the other coins looked authentic as they all had different amounts of wear and patina. Coin is 5.1 Grams. Width is hard since it's oblong, but one measurement was 23.1 mm.
"From a guy who got it at an estate auction." - not a good pedigree! Considering that and the stylistic similarities to the good quality fake linked to below, I'm going to lean towards yours being bad. http://www.zeno.ru/showphoto.php?photo=100005
It could also be that the blob from the cast defect makes it off weight. There is also no seam on the sides from a casting.
All genuine Chinese cash prior to the late 19th century were cast. But the absence of a seam is not in and of itself damning for these issues. That link says that most are around three grams - yours is five, much too great a difference to be accounted for by the flaw. A better idea is to hang on to it and learn from the piece, particularly if you are so convinced of its authenticity.
5.1grams? That's usually a 2 cash weight range. At that weight the coin should be around 25mm to 28mm in diameter.