JASMA aka Japan Automotive Muffler Association is a trade association not unlike SEMA. They work with member businesses to ensure their exhaust systems meet necessary standards for quality, noise and emissions regulations, etc.
http://www.jasma.org
http://www.clubroadster.net/forum/viewtopic.php?p=80657#80657
Simply having a JASMA certification label on your exhaust manifold tells us very little about it. You will need to share detailed pictures if you want help identifying the manufacturer or specific model. You manifold might be rubbish or rare or both or neither. What's most important in an aftermarket header is condition, fit, weight, power, and legality. Many RHD headers don't fit LHD cars. If it's rusted to hell then it's rubbish. It probably isn't all that rare.