JDMs are fun and inexpensive to collect. The Koufax also comes with a facsimile signature.
JMDs are not rare--I see them all the time at postcard shows.
I can't say the date other than to state the obvious--sometime between 1958 and 1966--since it is a Los Angeles jersey. You could also check the back for a zip code on the JDM address. You know that if there is a zip code it cannot be from the fifties.
I prefer to get them postally used. It is a great way to pin down the date of the card as no later than. I love this one, mailed from Brooklyn in 1957 and signed on back and later on the front:
