The wider, serif logo was introduced in the early '80s, when Fender bought Sunn. Don't really know why they changed it, maybe just a slight alteration to signify new management?
As the OP said, red logo was introduced in the late '70s at the onset of the Beta period.
To the best of my knowledge, late '60s was mostly black on silver, and the silver grill was popular, as visible on the (mostly) tube amps of that period.
Honestly though, I'm really not the best person to answer this...