Since one of the pictures includes a vented valve cover with an oil fill hole, we can assume the engine is newer than the truck, but a valve cover is a bolt-on item so that's not definitive proof. Brand new replacement engines called "Target Master" were sold by Chevy dealer parts departments at competitive prices to most aftermarket rebuilt engines, so if there's not a conventional ID number on the machined pad, it's possible you've got one of them. I seem to remember them having an oddball number like the one you've posted.
Jerry
