Stop in for a cup of coffee

So you are talking about the Polara?

toolmanmike said

Found it for the 64.View attachment 1715171576

Click to expand...

That's just what I said. One spring listed.
No heavy duty option listed in '64 dealer books for Plymouth or Dodge.
The 1970 Hamtramck Registry - 1964 Dodge Dealership Data Book - Index
Now if the order sheets show otherwise, then that's got more weight.