any keyboards with the labels that have the fonts all the same size, ie 1988 and older are built better. They are built with a heavier steel plate and have the support wires under the numpad + key and numpad enter key. These labels look like the writing were type-written where as the labels printed with what appears to be a dot matrix printer I find are lighter and the build quality not as superior(These tend to be 89 and till 93).
my 10/26/1989 has a label with same size, type writer font. this one does not have support wires under the numpad enter key. so if the label does not have type writer font, it is guaranteed to not be of the same quality as 1988 keyboards. however, if it does have a type writer font or is white label, that alone is no guarantee and it probably should be 1988 or earlier, as you said. my uncle had a 09/01/1988 and that did have support wires under the numpad enter. i sold that one off because i'm a clean freak and my uncle had stored his in the garage.
having said all this, i will say something i always believed but now especially believe due to sour grapes (lol - i thought my 1989 was going to be just as good as the 1988 since both are white label): having a slightly heavier metal plate or wires don't make that much of a difference to me in terms of durability or functionality. however, i do like my keyboards to be elite so i accept that the 1988s are best.
at the same time, i only want something that is "mine". i was old enough to remember 1990, 1991 so a keyboard from 1989 belongs to me (because what you'd see in the classrooms in 1991 was manufactured in 10/26/1989). however, the 301s from 1986 are at a time when i was 1 yr old and don't remember at all. not exactly the most logical reasoning but whatever makes me happy. i keep these M's purely for the nostalgia. they may feel nice but i don't type on them because the heavy key switches are not good for the wrists.