I bought this one for my CB550.
http://cgi.ebay.com/ebaymotors/FRONT-BRAKE-MASTER-CYLINDER-XS400-XJ600-SECA-SRX600_W0QQitemZ120436965251QQcmdZViewItemQQptZMotorcycles_Parts_Accessories?hash=item1c0a9a3f83&_trksid=p4506.c0.m245&_trkparms=65%3A12|39%3A1|72%3A1171|240%3A1318
Actually, I have the un-chormed version. I bought it, first and foremost, because my original master cylinder was wasted. I do like this one becasue it has a tilted reservoir, so it works really well with clubmans. The reservoir sits almost level. I also bought it because it has a slightly smaller diameter cylinder than stock, which I wanted. This is an important factor to consider. I don't know what the stock master cylinder diameter of the 650 is, but do check first. Using anything other that stock WILL affect the braking characteristics. It may not always be a bad thing, but it will be different, so keep that in mind. I have not gotten it plumbed yet, so I can not tell you of its performance or longevity. I had read good things about this cylinder on SOHC4.net, and that it can improve braking on the 550's. We'll see if it is actually true. The seller has good reviews/ as well, from others who have bought this master cylinder.
Later,
ryan