Hey Jayson

When you say "square headed nails" does that mean square or rectangular? literally square suggests hand wrought nails which are very old--could be 1700s. (But that's too old for square rule, I think). Rectangular head are typically 1800s. What about the shaft on that doubled-over nail. Is that tapered square or tapered rectangular?

I have found old barns here in Maine generally have lesser roof pitches. Big haylofts became popular in the second half of the 1800s. So a shallow pitch often means an earlier date (again, here in Maine.)

The absence of a ridgepole in a common rafter roof is another older indicator (generally).

And what about the roof trim detail? Is there a gable overhang or are the rake boards flush w/ the building? Flush denotes older here in these parts.

You haven't mentioned anything about the posts. The small plate suggests jowled/flared posts w/ an English tying configuration.

And I was thinking wet charcoal on those lines. I remember your experiment Tim... eager to hear if they're still visible.