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

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.