What is this arm, any arm like it from pivot back would not have old school axle mount like this. My guess is they welded stock RD axle mounts onto modern arm. Not a bad idea if you wanted to use stock mags. --- (https://i.imgur.com/v8qJkPK.jpg)
(https://i.imgur.com/7bxOlZj.jpg)
RZ350
Well crap didn't even occur to me box section with old style axle pulls, so obvious.