Source: TW
The Veda in Kashmir in 2 vols by HOS has CDs containing scan of a manuscript of 45 chapters of the Katha Brahmana. I have not had a chance to read them (and I do not read the Sharad script anyway) but according to the author M Witzel, these 45 chapters pertain to Grhya ceremonies completely. Even the surviving portions of the Katha Brahmana as edited by Suryakanta and others are very rich in Grhya Karma, so this makes sense. It appears that a significant portion of Kathaka Shatadhyayana Brahmana actually dealt with Grhya material. Significantly, the Charayaniya Mantrarshadhyaya states that all the Brahmana portions of the Katha Charayaniya Shakha were ascribed to Sumantu-Kashyapa. These Rishis are most commonly associated with the AV tradition, which as we all know, is particularly detailed in Grhya ceremonies.
Elsewhere, it has been noted that the Kathakam portion of Taittiriya Shakha is found appended to some Kapishthala manuscripts.
If you search the archives of this list, you will find some references that I collected on possible survival of Kapishthala Brahmana and Aranyaka manuscripts as late as 1970s. Not sure if they still exist. In this regard, there are numerous manuscripts of ‘Katha Samhita’ in Melukote which have not been examined (one of these is 200 folios or so), and also 3 of Kapishthala Samhita at Anand Ashrama in Pune that await detailed study as well. None of these were used for the fragmentarily published Katha Kapishthala Samhita. It is in the valley of Tapi river that one can do field searches to find any survivals.