Leisetritt muffler date

Does anyone can give me some info about the use of Leistritz mufflers under the early 1965/1966 cars?
Until the day of today I have never saw a early 911 with a Leistritz muffler with als a X/65 or X/66 date behind the part number.
Most of the time it's a Dansk, Bisschof, or Leistritz from the early '70's.

I am wondering if there is a reason.

The most early I found is a 02/68 date.
Attachment 417937

Does anyone have some pictures of a Leistritz on their 65/66 car?
Or can tell me something about the dates on Leistritz exhaust?