Stefano,
Someone may know better than me, but just from my own motors that I have personally owned, there seems to be no rhyme or reason to the 052, 053 blocks. The production ZL-1 blocks were all 052 I believe with screw-in freeze plugs and were NOT siamese bore...(all that I have seen)...I have read in books that the Yenko blocks were 053 blocks with push-in freeze plugs, although I personally have owned Yenko blocks that were 052 blocks with screw-in freeze plugs. And have owned 052 blocks with push-in freeze plugs. So I believe the 053 blocks were never used in production, but I'm not sure if there was even a difference other than the cast number. And I have owned tall deck Yenko blocks and "normal deck" Yenko blocks also...
The bottom line is that I have personally owned blocks that are contrary to what all the books I have read say.
Hope that helps a little...
