A number of the ones I've seen do not have a number on them. Your later model alternator will have the rear cover indexed 120 degrees differently so the post and connector are differently located than the early model trucks. Except for the Bosch, most aftermarket alternators have a difference in casting of the cases, which may be the tell if OE or not.

Anyway, here is what I have between 110a OE, 140a Bosch, and the L-N just because it's from another story.

Attached is also a close up of the back of the 110a.

 

And the weight different between a 110a and 140a with its larger windings, another way of checking the alternator output. Copper and iron are heavy. I'll include the L-N 230a too.

 

And found this, supposed to be an OE '06 140a.

 

I am thinking so far it's not the original alt. Of course it's still installed so can't weight it. I took a couple pictures to see what you think.One is from the rear. I did notice that the main 12 volt cable attaches in the upper right rear corner and the plug in the lower right rear corner looking from the front of motor. In the pics you sent so far that's different.

 

That the different cooking of the alternator. Not a hard thing to change.

It's a motorcraft by the case numeration, doesn't show a rebuilders label. A straight on view might be more telling of the amps but the case size.

My voice box has been screwed for a while so little voice over.