I have a 455 #396021F - 68-72 block from what I have read....but the VIN derivative stamping pad on the drivers side front that is suppose to have a bigger number has only "6243"

Any idea what this 4 digit number is? I'm trying to figure out what year block this is.

 

First, don't believe everything you read in the FAQ. While the "F" blocks were primarily used 68-72, there are documented blocks with VIN derivatives from 1973-74. In your case, the sans serif "F" (as opposed to "F") would indicate a 68-70 block. The drill spot and non-standard stamp indicates this engine was not originally installed in a car. It could be from a marine or industrial application, or it could have been a replacement block. Other than it obviously isn't numbers matching, none of that matters, as all the block castings were the same.