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4.6.75 is the actual casting date that was made in the sand for blocks poured that day and the 76 would be the model year and 76 models came out in the fall of '75 so blocks would have to be cast in '75 (& on into 76). No prob there. But the Mopar literature is notoriously inaccurate on the start/finish(s)of their (model year & other) stats as I had a '75 Cordoba that had a 4xxxxxx 360 original engine in it & I was expecting a (3) for the thin wall. The change from (3) series (before 76) to (4) denoted an overall change to the so called "thin wall" casting technique (big and small blocks) but susequent testing proved that alot of later thin walls were as or even thicker than the earlier blocks (See Andy's sonic testing article in the archives). Other than a 30 over DD build, sonic testing is the smart way to go. Cannot assume anything. On the suffix(-4) everytime they changed the mold cause it shifted or enough sand got used up from sticking to the block itself or another prob of any sort they changed that # to the next digit up. I'm assuming they started with 1 rather than zero and a lower # I believe is better as I'm not sure but I heard that the mold quality deteriorated w each change but being practical if it sonic checks you're set. for a high output I'd water pressure check/magnaflux also

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Thanks. I had no idea about the -3,-4.

Learn something new everyday on Moparts.