Papyrus 28 (= P.Oxy. 1596) containing John 6:8-12, 17-22 formerly Berkeley, Pacific School of Religion, Palestine Institute Museum, Pap. has been sold to the private collector Gifford Combs, Los Angeles. Thus, it can be identified as "Collection of Gifford Combs, Los Angeles." (HT: Hugh Houghton)The first editors, Grenfell and Hunt, dated the papyrus to 300-375 CE. Nestle-Aland gives the date 200-300 CE, whereas Orsini and Claryssee date it to 250-350 CE.
Editio princeps: B. P. Grenfell & A. S. Hunt, Oxyrynchus Papyri XIII, (London 1919), pp. 8-10.
IGNTP transcription is available here.
Wiki here.
More about Gifford Combs here and here.