It is the serial number "prefix" (1st three digits/letters; 6NZ) for a C-15 CAT engine, built roughly in 2000-2001 range (might find it in some 2002 truck, also)

{single turbo, easy to work on; improvements in front and rear covers, over the 3406E models, and weighs less, also}

Edit: I heard the Omaha CAT dealer has built roughly 8 glider kits, with Reman CAT 6NZ engines, and reman transmissions. The foreman says it is a nice combo, with a new truck glider kit.

(CAT 6NZ engine _right side.jpg)

(CAT 6NZ engine _left side.jpg)

CAT 6NZ engine _right side.jpg (20KB - 91 downloads)

CAT 6NZ engine _left side.jpg (94KB - 102 downloads)