Questions About Airmail Cover

Hi,

This postcard was sent from my Great-grandmother to my Grandfather and I am looking for more info about it.

Why is the name Glenn Curtis added to the front in white letters?

What does the number on the front lower right mean (it's either 1329 or B29?

What is the best way to research an old email cover, and how can I find out the value of this postcard? It's an important part of family history so I'd never sell it. But, I do want to learn more about it. It's also of particular info to me because Curtis is an old family name, although no relation to Glenn Curtis that I know of).

Thanks,
Curt