I though I had a LT1 Manifold casting #12552138 that I picked up a couple of years ago Does anyone know if its for a LT1 or not. Thanks

 

That is one of the LT1 casting numbers for the intake manifold. GM used 4 or 5 different casting numbers to differentiate between years and motors (LT1/L99). The only real difference is the 92/93 castings which had a different fuel cross over.

 

That may be a "late" intake casting #. The original intake from my 94 is 10108630. This source lists it as a later model. As noted in the post above, they are basically all the same.

LT1 vs LT4 Intake Manifold Differences