"The Schwinn Tiger was made from 1955 to 1962 and unlike many Schwinn bikes, the Tiger pretty much stayed exactly the same throughout its run."
D+5 numbers would be 1957 for a Tiger. 6 numbers would be 59 as noted above by hammer mechanic.
57 model:
View attachment 224871
Also noted above, the chainguard isn't the same style as a 57 as shown in the catalog, it's the later style like a 59 . The Tiger is shown only available as a 3 speed in 55-57. In 59 you could get it in one, two, or three speed. So your one speed S-7 wheel points to 1959 also.
59 model:
View attachment 224872
Lastly, the handlebars look 59 also, the earlier model has curvier bars, yours look more like the longer ones on the 59 illustration.