...yes, thank you. I think I understand that issue. I'm still puzzled why slicing the seat tube in the rear seems to be such a popular construction method, and why you see this sort of failure so rarely on bikes that get ridden pretty hard and put away wet. Upon inspection at initial receipt, the ears of the top lug were definitely drawn inward, in the manner you sometimes see with an undersized post fitted to the frame.

My speculation (and it is only that), is that in this case, the thin frame tubing walls of a higher end frame tubing set, coupled with the fastback stay cluster design, and too small a seat post, gave just enough flex in the area of the bottom of the slotted tube to initiate this failure. It also looks like whoever built this frame did not do much relief rounding at the lower end of the slot. In retrospect, this seems unfortunate.

My hope is that with some relief drilling and a well fitted post for support, it might be either arrested, or at least become one of those failure modes that can be ridden for years without much danger. Certainly the clamping action of the seat lug on a properly fitted post is what makes this design possible in the first place. Ah any rate, it should be obvious if this was a bad surmise on my part well before any sort of catastrophic failure occurs, wouldn't you think ? Or am I mistaken ?

We do have the earlier post in the thread by someone with a similar tear, who has done a similar thing with success. But I'm guessing from his description of his frame he has a more traditional seat stay attachment.

I always wondered about those wrap around stay attachment designs, as to purpose. I guess this might be one reason for doing that.