This was one of the earlier routes I drafted, and I went by the "We don't use exit numbers on the US highways" rule. State routes weren't a glimmer in anyone's eye yet, and I really only expected to use I-50(666) style labels on interstate duplexes.
I think this one should get NH16(666) style labels. I'll add this post to my ToDo list, but probably procrastinate about it.