It looks like the Nissin master cylinder I have on my CB550. Mine also says 1/2 on it, but mine has a mirror mount on it. It came from a 2009 Kawasaki KLR 650. Lots of bikes used this master cylinder. If you search Ebay you can probably find a very clean used master for cheaper than you can buy a rebuild kit. My master was from a very low mileage bike and was in perfect condition and very clean for about 25 dollars shipped.