Re: Johnson, year etc....??
That is definitely a 1958 Johnson RDS-20. First year for the outer housing on the midsection, the two line water pump, and fuel pump. Yours is the short-shaft model. The long shaft is RDSL-20.
Great little outboard, and way lighter than it looks...about 150 lb.
As for its value, if it's in perfect running condition and excellent cosmetic appearance, it can be worth more than you'd think. It's one of the prettiest of the Big Twins.
I sold mine on the spot at a lake to a guy who had a 1958 fin boat. Took it off my boat and put it in his truck. $900. That was $200 more than I paid for the boat, motor, and trailer, and the boat was pretty much junk.
Don't expect that much, but that model is in demand by the classic boat crowd, if it's in perfect condition.