Mast Sheave Material

[akswimdog]

Greetings, Hopefully my attempt to upload a photo will work...I have 6 mast sheaves originally installed in 1974 that I'm attempting to replace. Can anyone identify the material (based on the photo if it uploaded...) used to manufacture these sheaves. I'd like to get some kind of baseline on what's been on the boat all these years...It actually looks like they haven't worn all that bad.

The second part of the question is; What's the best type of material out there now that I should be looking for to replace these with? Many thanks in advance for any insight on this.