Apparently many "inexpensive" gun safes, for example 250lb Sentry G-4311, are classified by CA DOJ as "lockboxes". And certified lockboxes require a receipt for affidavit to be accepted.
# Is presentation of a receipt always required when a person signs the affidavit stating ownership of a DOJ-approved lock box?
Yes. If the firearm recipient claims an exception to the FSD requirement due to ownership of a DOJ-approved lock box, a receipt for the DOJ-approved lock box will be required in all circumstances.
http://ag.ca.gov/firearms/ab106faqs.php#4
Now I'm wondering what would a person buying a used such lockbox provide as the receipt
P.S. I've stepped into this trap just last Sunday at Martin Retting in Culver City.