Documents also need to be verified and linked to the item

A period receipt, certificate or letter can be very useful, but do not treat it as evidence independent from the item itself.

Check dates, model description, serial number where present, letterhead, signatures and consistency with the item’s physical characteristics. Generic paperwork referring to “a guitar” does not necessarily identify the guitar in front of you.

Keep high-quality scans or photographs and, where possible, the originals as well.

The strongest documentation is what fits the physical evidence, not simply what tells the most interesting story.