I'm only interested in a 99 of that era.
I have enough 66's
1909 66 back clamp Lotus
1922 66 back clamp Red Eye
1923 66 side clamp Red Eye
1928 66 Filigree - old style bobbin winder and electric hand wheel (hard wired into cabinet w knee control)
1945 66 Godzilla black side - new style bobbin winder w back tack, has bakelight cord plug, would have come with knee/foot control hardwired in cabinet.
I don't think Singer made any significant changes to the 66 after the back tack.
I have a 1954 99k with the single grove lever, it's a lot like the 1945 66 (Singer stopped making the 99 in the US around this time)
I also have a 1956 99k with the double grove lever. This is the last style 99k made before they went to the Green type.
So I really want a 99 with the old style bobbin winder.
However I already have a 1925 128 in a bentwood case with the older style electrical connections and the knee bar with the round connection. So I don't want a 99 with the same case - which the oldest model I've seen with the new type was from 1929, though most are from the 1930's.
I also have a 1922-127 and 1922-101, so I have more than enough Singers from the 1920's
I don't have a lot of 15's, but I don't know if they made that many changes to them over the years (and different lights don't count!) I have a 1914 treadle, a 1945 15-90 and a 1952 15-91.
From what I've read, the big different between the early 15's and the later is going from a 1 o'clock bobbin case to the 11 O'clock case. My treadle is 1, and my 15-90 is 11. The main difference between the 88, 89, 90 and 91 is power source, and the hand crank, treadle and belt driven motor models are all the same basic machine - only the 91 with the potted motor is significantly different. I don't consider a different flywheel to be significant. I've got enough examples of a Singer spoked flywheel in the house . . .
From looking at the pictures, other than the face plate and tensioner, there is no significant difference between a 1936 15-91 and a 1954 15-91. Though I may consider a 1930's era 15-30 (this would be one of the last made in that sub-model) and I would grab a silver one if I could find one affordable!
I did find one in a quilt shop on display, but it was in such horrid shape that DH would not let me make an offer on it.
So you can see, I'm really looking for a very specific machine to fit a specific spot in the collection. There was one in Burnsville yesterday, but the listing is removed today .