DIY beginner - What search terms do I use to learn how to rebuild this 1/2 in old backflow prevention 3-way for water pipe to boiler?

[bonjour]

Hi, I'm a beginner. DIYer.
1929 house, Foursquare.
Steam two pipe radiator boiler system.

3-way backflow preventer needs to be rebuilt (I believe) or replaced.

I prefer to rebuild it.

How do I rebuild it? What search terms do I need to use to look this up on YouTube or this forum?

 

[bonjour]

My other photos are too large file size to upload. What is max file size for forum?