I picked up both the 1961 "rude black" 601xx and the indigo 1951 601xx. As you've seen, I have the former up in the sale thread, since I found the waist to be more constrictive.
Curious about the waist stretch you've noted, however, since that's the only aspect I didn't like about the RBs. Is yours just because you're starting a size up, or have you noted the waistband to have a lot of give?
Between the two, I found that
- The 1961 RB has less give in the fabric
- Waist sizes are similar, if not the same
- 1961 has a fuller seat, with some hip flare. If it weren't for the waist, it's actually quite comfortable in the top block, although definitely diaper butt.
- Rises are about the same, but noting the difference in seat
- About similar in leg and hem, with the 1961 RB maybe being a bit fuller