The sending units changed over the years.
In part this may have been because the fuel tanks changed.
We also know from Redfish that the gage unit calibration changed - I forget the year. Search will turn it up. Also sometimes the gage itself would be tweaked or the hash marks on the gage placed in the right spots in production.
" In the mid sixties to the early seventies the part number was 2421493.
From around 1973 to 1975 or early 1976 the number was changed to 3621515.
Then Chrysler changed the number to 4051004." posted here
Part numbers I looked up were:
1967 p/n 2257 493
1973 p/n 3621 615
Which doesn't quite match with the numbers Impreza wrote (but I think he was working from memory)