302 Swapped Block Casting Mystery

I've recently bought a '66 mustang coupe, it has been 302 swapped. I was trying to verify what year 302 is in it so I pulled the starter off and found that the block casting says E7 E the third digit appears to never have been cast. I understand that the E7 means its from an 87 but now I'm curious if anyone knows why the third digit would be missing. The area where the third digit should be has the same appearance as the rest of the block so I don't think its been filed away. I also have the partial vin from the flat on top of the block it reads hf165129 if that helps. the full casting on the block is a large 21 then below that the E7 E below that ca and below that a large S. Thanks in advance for any input.

Nothing like that on the block. The only casting marks I could find are the ones in the photo, and the engraving on top of the block in the back. That link you sent is what I was expecting to find. The guy I purchased the car from didn't do any of the work to it so he wouldn't have any info on it. Who ever done the swap done some things that I'm gonna have to redo in a more correct manner, I've already started fixing some of it. I plan on going efi in the near future and using a power adder a little later probably around mid summer, so right now I'm just working out bugs and trying to see what is there to start with. I used a boar-scope into one of the spark plug holes, the pistons appear completely flat other than valve reliefs.