The 2 cent stamp is perf 10.
There is really no doubt about the 1916 on the receiving postmark. (Also, the Chicago Main P.O. would never be lacking for correct year-date slugs.)
And the clear gap on the lighter strike of the handstamp on the front:
A heavily-struck, smudged, or overinked 6 can look like an 8.
I believe this is a 1916 use, thus E9.