How are issue dates known?

Hi there,
SO impressed by this amazing catalogue and community!
I’m not sure if this is the right place to ask this question but I was wondering how we know when certain postcards were issued? Is this information from the RT&S catalogues? In particular, I’m interested in those cards featuring images of Queen Victoria. I’m keen to know if they were issue pre or posthumously?
Many thanks

@KimCG Welcome to the forum!

Some catalogs have a date on them but the vast majority of dates on our site (First Use) comes from the first postal usage, for example:

AFAIK the are no records from Tuck directly because their offices were destroyed. If such records exist we would love them.

Here is a item with a set cover (packet cover), but I don’t see a date on it.