I remember that as well. It's on the block under the header opposite the starter. If i remember, the last digits of the VIN number are stamped here.
Of course i have never seen it myself, so no idea if it's true.
Other than that, the only numbers would be date codes, and all the parts for an '87 should begin with an E7 casting number. An E8 or E9 would have come on an '88 and '89 and would not have appeared yet. The block i beleive would have an E7ZE casting number
I'm not so sure they updated the casting numbers from E7, because the castings were exactly the same, so they would've had to change the cast in order to change the number after the letter. So if that makes sense, an '88-89 and so on would still have E7 heads because they never changed the castings. That should be true for the blocks as well.
So, if that's true, then i would assume that pretty much every number on a '90 would be an E7, so would be the same as any other '87-93 as far as what the numbers begin with, so you wouldn't be able to tell what the car actually came with....just that it had the right E7 and not an E6 or E5, because the '86 and '85 were completely different castings.
i found it, this is thread i was talking about.
http://forums.stangnet.com/showthread.php?t=623954&highlight=VEHICLE+Identification+Number
http://forums.stangnet.com/showthread.php?t=623954&highlight=VEHICLE+Identification+Number
Sorry,I didn't read the link because I was in the process of posting at the same time.