First PPK chambered in 7,65mm was introduced in 1930, 9mm kurz followed in late 1933.
Walther ZM made the DURAL versions between ~1933 - 1943, chambered in both, 7,65mm and 9mm kurz. Early guns has got the old (open) frame design, later versions got the reinforced (closed) frame design. I do not know about any deform/break.
Anyway, Walther/Ulm decided to reintroduce the PPK/L only in 7,65mm and .22lr.
Martin: What was the actual frame difference between open and closed PPK design? (I initially thought you might be referring to the difference between PPK and PPK/S frames, but on second reading of your post, I think your context was only PPK.)
The design difference is visible on both, PP + PPK. You may look at the illustrations in E.J. Hoffschmidt's book 'Know your Walther PP & PPK Pistols', also shown in Dieter Marschall's book 'Walther Pistols, Models 1 to PPX' . The so called 'closed version' was used by Walther/Ulm as well.
In addition: The design change was done on both, DURAL and also steel frames. The cutout for the clamping piece (and the piece itself) was/were redesigned.