Purpose of this thread is to focus on the change from the early square corner sill plate design to the rounded outer corner versions used later in 65 production and through many of the following years. We've had earlier discussion but those were not well focused. Of course these were often replaced over time or when the car was restored and since the service replacements were made in the new style this can add to the challenge of determining when the change took place and I use the square style (since those are rarely added to a rounded corner sill plate car) as a guide and carries more weight in the interpretation.
In an earlier survey of collected data from my pictures I came up with the change over taking place around and between the 5R175262 to 5R180051 production period
Examples
The first/early design
For coupe and fastback
For convertible
++++++++++++++++++++++++++++++++++++++++++++++
Later
Coupe/fastback
Convertible
As always, we invite other reports and findings. Please keep to the plant and year