It is generally believed that at the time, only one double chambered for the .577 Rewa was made. I'm sure that a number of recent rifles would have been chambered for the .600/.577, the guys over on the big-bore forum at AR bar-up over this kind of thing.

No doubt Ross Seyfried has a six-pack of Ruger No.1s so-chambered!

I reload for the .600/.577 'Short', otherwise known as the 20-bore/.577 Express, and successfully employ Kynoch(Bertram) Rewa cases shortened by 1/4-inch.

Most of the current Rewa cases would go to cartridge-collectors I suspect.