The difference between when most of us started fooling with old guns, and today, is the internet.
If you google "What load should I use in my L.C. Smith shotgun?" the first link on the list is this
http://www.lcsmith.org/faq/loads.html
Knowing the author thereof, I think it, and the other FAQs, are helpful. And if the gun owner has more questions, the LCSCA Forum does not require any sort of registration.

We have this forum, and Shooting Sportsman, and Shotgun World.
And collector's forums for Parker, Lefever, Fox, and Remington shotguns.
Guys even post here looking for information about DARNEs!

Lots of (mostly good) information out there if someone is interested in looking, including documentation of turn-of-the-century loads
https://docs.google.com/document/d/1F2sQuPm05IE4VWYYnCkvuXmYEzQoWd_SQgaAfUOZEFU/edit

But folks can do what they want...and do...sometimes with bad outcomes.

https://sites.google.com/a/damascusknowledge.com/www/home