This source http://www.astrolojidergisi.com/yazsaati.htm has researched the time zone history of Turkey, based on newspaper archives and official documents. All goverment documents in Turkey since 1921 are in a public archive, called reszmi gazete http://www.resmigazete.gov.tr/ The source http://www.astrolojidergisi.com/yazsaati.htm has at the bottom a link Ekler, which I believe means 'notes', and which leads to screen shots of documents. There are several pages of screen shots http://www.astrolojidergisi.com/yazsaati-ekler-1-40li-yillar.htm http://www.astrolojidergisi.com/yazsaati-ekler-4-70li-yillar.htm http://www.astrolojidergisi.com/yazsaati-ekler-5-80li-yillar.htm My problem is that I do not understand Turkish. To me, the source seems believable and well researched. I think that TZ zone Europe/Istanbul needs to be revised.