Unknown Buick Engine Number

[Jordan0051]

Can someone please fill me in on what engine I have?  Here is the info listed on the site....

Engine Number Format: A-B-C-DDDDDDD

Where:
      "A" was the model code (0="Special", 1="Special Deluxe", 2="Skylark")
      "B" was the model-year code (H="1961", I="1962", J="1963")
      "C" was a single-digit plant code
      "DDDDDDDD" was the production sequence number

My engine number is: 3-J-1-597340.

Any ideas what the "3" would indicate?  It's supposedly out of a '63 Skylark. Maybe a replacement engine??

Thanks in advance,

Jordan