It is also hard to judge ANY combination of machine and case after 75+ years. Folks always seem to claim that items were original, but when pressed for details it usually turns out that its was that way "as long as we've had it" and "the original owner told us it was..."
I had a two day discussion (read: argument) with a seller that insisted that their 1875 Singer 12 with a motor came from the factory that way because that is what their grandfather told them....
Mickey2 is correct, there was no "cutover" point for most of the items (machines, attachments, decals, etc) made by Singer. Travel from one side of the US to the other was slow and stock could not be wasted, so new things were made available and the old stock would be available as well, until it ran out.
EDIT: What you CAN do however is research the date of introduction of the new item