My eyes aren't that good, but is that serial number in the high 3 millions? It will likely be late 60s? More pics would help. No id tag on the swivel bracket?
Yepper She's a 74 6hp. Those are fine little engines and have been around since 1965. (Based on older 5 hp iterations before that) In fact , you will find that this particular engine has changed little in over the years, so you will be assured a good parts supply. I had a 1965 Johnson 6 and it ran beautifully, sold to a buddy who still fishes with it on a small lake in MA.