Re. the narrow tyre clearance at the back - my understanding is that it was considered difficult to bend the stays on Reynolds tubes. The alternatives are a narrow rear tube (given the riders background a non-issue) or extra long stays to get round a fat tyre resulting in bargelike handling.
Not being thus constrained was an advantage for US builders and allowed them to gain a reputation as being able to build better handling bikes. If anyone can deny/confirm this I'd be v. interested
Handling for what? short stay work well for climbing, long back end with loads of clearance can be flexy. i am not sure hwat tubing they used as its a service de course frame. so he could choose what to (753,531,653 somethimes soem columbus)
i can say that the chainstays on my roberts are a bit tight at the back and it does have a short chainstay and does handle well for climbing and single track. i have has same era US bike that have more clearance and are better trail/ dh. but it hase a 73mm bb shell. it is in part a component thing as well. the old kit was often road based 68mm bb shell not 73mm so this made even less space to fit all this in.
I am a mad fan of this bike. I loved it then and i still love it now.
I am trying to build a replica frame from scratch I can tell you it is very easy to bend the tube, the problem is that is soo thin it gets out of shape. 753 is way to tricky for me to mess with. I have been looking at alternatives 531 chain stays which come pre bent. I am pretty sure roberts used a road chainstay and just were very carful with the set up and maybe just a small tweak at the bb end. anyway i have tried twice to replicate this frame and it is really dificult. it looks so simple but you have to hand it to roberts they did a great. job. I guess that the difference between and have a go hero and a master craftsman. this frame is not built its crafted. still loving it.