GM has been using the 3 letter designation for years. All engine in GM vehicles will begin with the letter "L". This applies to all gas and diesel engines. The same is true for all transmissions, the only difference is they start with a "M". The "L" designation for engines is piece of history for GM. On rare occasions, they reuse the RPO, but it is very rare. A lot of GM vehicles have been identified over the years by RPO. Some of the more famous ones in my opinion are Z71 - used on 4x4 trucks, ZR1 - used on HP Corvettes, Z28 - used on Camaro's. There are plenty more. Hope that helps.
2006 2500HD LBZ 4X4 CC/SB LT3, 265/75R16, Rhino Liner, PCV Reroute, Resonator Plugged, 35w 6K HID Low Beam & Fog Lights, LED Interior Lights, ISSPro Pyro, Boost, & Rail Pressure Gauges in A-Pillar, Cognito Idler/Pitman supports, more to come soon.
1996 K2500 Suburban 7.4L, 265/75R16, Brush Guard, Nerf Bars