In my mod I use some era letters that the 1.3 adds on to the game. The problem with using these is anyone who does not have the 1.3 won't see the eras in instant action. My question is what eras can I use without requiring people to download the 1.3?

I don't think any besides CW and GCW (c and g).

Just ship the 1.3 lite installer with your release, that'd be easiest. Though, you might wanna confirm with Zerted first.

If people want to bundle in the v1.3 r129 installer that's fine with me. Just make sure it's r129 (the version without the .bat menu system).

I'd also prefer if people tell me what they're bundling it into as well. That way when users come to me asking for help I know what they're talking about. It also lets me get in touch with you if there turns out to be any important flaws in the bundled patch.

Alternately, you can use the era renaming feature (change={} tables in the addme.lua) to rename the C or G era to whatever you want it to be. Users without v1.3, will see the normal C or G era while users with v1.4 will see your customized era. The draw back is if you have more than two custom eras.