TA blocks were the
original HD/race blocks before X-blocks came out late in the '70's(?), around or after W-2 heads. However, Direct Connection did sell blocks through their program which may have been production blocks. But the only difference from the factory OEM blocks appears to be "
stress relieved". It's been almost 50 years but I bought one through Massey Chrysler Plymouth on Beach St. in Daytona Beach, FL. (Note the prices!) However, I don't recall it having any TA or X-block features. Based on the catalog wording/descriptions, I would think the 340s were a factory blocks along with the others. I suppose that the OP may have one of these. But I can't think of a way to prove it without documentation, features or part numbers on the block. Either way, it would be another OEM part assigned a P-part number and sold by DC.
Same listing in 1977 but Nov. 1979 shows the "NEW" race block with 2 or 4 bolt mains and the rest of the blocks listed as "production".
Glidden probably got the first ones.
1976 catalog:
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