It's a company that makes shift kits
http://www.txchange.com/sk.htm#Import
Here's the one you would use
SK® RE4R01A
Nissan RE4R01A, RL4R01A Mazda R4AX-EL, Subaru 4 speed ECAT, Nissan – Infinity – Truck - SUV V8 RE4R03A, Isuzu Diesel JR403
1988up 4 speed with or without lockup
Corrects/Prevents/Reduces:
Planetary burnup; 3rd clutch failure; forward clutch burnup Soft 1-2 shift; 2-3 cutloose; Band burnout and low lube flow causing overheating and total burnup