The dates and methods of identifying "vintage" cherry switches aren't really set (logo used to be the easiest tell, but people have found some not so great, newer-feeling big logo switches) but the dates you list are ehhh. MX browns weren't even introduced til '94, so vintage browns had to have existed at least that late, but most vintage blacks come from 80s and some early 90s boards. Then again, people have pulled great switches out of some mid/late '90s boards. While I'm sure it has lots to do with molds and material, I'm also sure that decades of use contribute as well so it's not really an exact science anyway.