Half the letters in our archive arrived undated, and most of them can still be placed within a season. Paper stock, ink colour, postmarks, and one telling turn of phrase do most of the work.
Let the postmark argue first
Cancellations, stamp issues, and even the price of postage narrow the window fast. One “orchard press” letter we argued over for a year settled in an afternoon once we priced its halfpenny stamp.
Phrasing is a fingerprint
“Your loving sister” versus “yours” marks different decades in this family’s hands. Keep a list of each writer’s habits — the archive’s reading room holds ours, and it is free to any cousin who asks.