Found a Jap website with something interesting on it... You'll have to do a bablefish translation on it if you want to decipher it though.
http://www.lcv.ne.jp/~yuki180/...i.htm
Basicly it looks like a history of the 180sx. Here is the section that it seems to mention a "Type X Nismo" 180sx
"Special specification car "TYPE X NISMO" "TYPE R SPORTS" sale
Strut tower bar & sport muffler side sill protector of NISMO make were equipped especially in TYPE X NISMO.In TYPE R SPORTS, the sport muffler audio & the diver city less of side sill protector strut tower bar NISMO make were equipped especially. The platinum white pearl sets as a special paint color."
Anyway... that's my discovery.