It is a KS (
KindShock) brand shock. Appears to be a model KS-291 - - rebound adjust only
(well, besides the spring preload):
More info here:
http://www.kindshock.com.cn/en/produ...7&fid2=&id=693
BTW, I see that you are looking at a Cadillac bike. I would be a bit leery of that.