Look at your manufacturing plate on the door jamb and see if your truck was manufactured in 1990 as a '91 model. The '90 models didn't have EGR, so maybe a few early '91s were made without it.

The second possibility is that the engine isn't original - it looks fine, but I haven't seen valve covers that color.... maybe just a repaint.

The exhaust manifolds appear to be from a '90 engine, because the exhaust manifold flange for to the heat tube is missing and would normally be found where the cutout is on the heat shield.

Frank

'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum