Quite interesting-- I had always assumed that the kk53 had a nickel membrane, like the kk54. I didn't realize it was aluminum! Do we know the history of why they chose the different materials in different applications?
I do not really know the history but some bits. There were SDC-mics developed by NWDR and manufactured by Hiller (M59,M60,M58), that used aluminum as membranes and these came out 1953. Furthermore, Hiller made models of their own (M159,M160), that used the same capsules, but a bit later, like 1954 on. The M50 (M49 too) was also developed by NWDR already before.
They must have found out, that aluminum gives excellent results, as some of us have been able to witness until today. (Aluminum membranes) work well, not only for omnis, but also for cardioids (M59,M159).
I have compared the Hiller omni-capsules in M60 and M160 with K53. For a while I mistook them to be the same capsule, but quite different capacitances revealed it not to be so. They are exactly the same size and structure, even inside, but although the backplate-pattern is the same, the K53 has slightly bigger holes. Hiller capsules have the membrane retaining-ring with screws sunk in it and k53 has the screw-heads sticking out. The Hiller omni is 50pF and the K53 is 90pF, so there is a difference in the gap too.
I believe choosing to use aluminum was due to high quality special processed aluminum film becoming available and was surely at least partly chosen for the reasons Klaus mentioned. The pitfalls however also soon became obvious: The omnis needed much more tension and any fault in the material was fatal, leading to problems tensioning the membrane.
I have no proof, but the original provider must have stopped manufacturing the foil needed in that quality, and others failed to produce that. Perhaps that is what led to the use of nickel in K54? I don't know how long the production of KM53 went on, but my understanding is that it was at least some years. My understanding is that K53 was used in M50 only a short time?
Hiller was also replacing aluminum with HostaphanŽ (a brand of polyester, KH) in the M159 capsules during the final production period.
Another curious detail is, that the last Hiller-mics were using AC701 instead of their late flagship MSC4.