I've never encountered one like it. This is on a rifle a friend is considering buying and is having me look over.

An image search for "Custom Mauser 98 safety, left side of shroud" turned up only one very fuzzy image with no other information:

and it isn't exactly the same. I'll get a look at it this afternoon.

Thanks,

Rex

Looks a lot like the factory safety on the Brno 21/22.

That is correct. The Brno 21/22 came with that type of safety. I have seen similar conversions made to standard Military style shrouds.