In 1949-50 Johnson 10 HP that seal was used for the carb’s primer plunger. In ’51-’52 it was offered as an assembly of o-ring and retainer together as Frank’s picture shows, and then in ’53 the o-ring and retainer were listed separately.
I haven’t been able to confirm it but I believe that fat o-ring is the same one as the extra one that’s included in many of the carb kits for the 50s motors, even though those carb kits don’t apply to those earlier years.
![1949-50-carb-seal](https://www.aomci.org/wp-content/uploads/2022/01/1949-50-carb-seal.jpg)
![1951-52-carb-seal](https://www.aomci.org/wp-content/uploads/2022/01/1951-52-carb-seal.jpg)
![1953-carb-seal-and-retainer](https://www.aomci.org/wp-content/uploads/2022/01/1953-carb-seal-and-retainer.jpg)