I don't believe many service blocks have the casting number on them. Service blocks were over the counter replacements and would have had the same characteristics as original blocks. If the block doesn't have the C or A scratch on the back it may be a bored 390 block. The 1U crank is a early 428 crank and would not have come in a 69 engine. You should have had a 1UB crank in a car that late.
Pop the center soft plug on the passenger side and see if 428 is cast into the water jacket on the floor between the cylinder liners. Also, look at the oilfilter boss and post the date code.
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