My best friend's family when I was a kid had a bunch of the Safemark 3 ribs...two-twenty and 4630 both had them.  I remember they had real flat, meaty tread on them.  I wouldn't be surprised if they are still on the tractors...the 46 has been beached for quite a few years now and the two-twenty has had an auger mounted on it since the '80s.