I had a very similar question regarding a block I purchased. See the post on this forum entitled "Do these 302 DZ Block Numbers make Sense?". After some discussion, it was determined that the block is "the real deal", although I never figured out why there was a 4 month difference between the casting date and the assembly date. It would probably help to post a picture of the casting number and casting date for the experts to help.
Best of luck,
JmcBeth