Simply cleaning the lock didn't fix the sticking latch and there was the issue of the rogue broken spring. Time to learn more about the lock.

The lock casing has the brand name Russwin and inside many of the parts have P1213 stamped on them. So I started with the assumption that this is a Russwin P1213 mortise lock.

My goal was to find anything I could about this lock. Especially pictures of what the parts inside the lock should look like and places selling replacement parts. The internet has everything, how hard would that be to find? It turns out the internet is more limited than I expected.

Eureka!

Doing searches for "Russwin P1213" turned up a lot of online auctions selling old locks and exactly one blog post with a teardown of the lock.

That blog post entry was like hitting gold! The author posted several critical pictures.

(don't mind the arrow, that's from the blog post)

Do you see the spring in the upper right of the picture? That's the broken spring from my lock! Now I know what it's supposed to look like.

Other resources

Now having a slight taste of success, I continued digging even further and looking for anything I could find about this lock. Old replacement parts for sale, old manuals, anything. 

A site called Historic Houseparts has an entire section for salvaged antique door parts. Unfortunately, only a couple parts in their catalo look compatible in this lock, and no matching springs.

However, their Lock Resources page is a wealth of knowledge of old locks and manuals. Which led me to a product page from 1899 with a picture of my mortise lock in it (bottom left).

This lead me to Archive.org, which has a full copy of the company's hardware catalog from 1926, also containing a picture of my lock.

Great, now I have a few resources to work with. First, let's see about that spring.