TOPIC: 1966 Johnson Super Seahorse 40 hp

I am considering buying a 1966 glass boat advertised with the Johnson Super Seahorse motor.
I find this motor came in three models:
RD28 RDS28 RK28. If anyone knows the difference between the models or any issues I would appreciate it. If I buy, it would be used primarily in brackish and freshwater..and occasionally salt. Are any of these models okay for brackish or saltwater if properly flushed?
Thanks,
John