The following is just my theory:
SEK converted the Ητ livestock carrying wagons 8601-8612 (Athens Region) and 8613-8650 (Salonica Region) made by Ringhoffer in Czechoslovakia in 1926/7 into excursion carriages, sometime before WWII. These Ητ wagons were the only ones with 6.5m distance between the axles in the Greek stock list of the time. Also they had a characteristic V strengthening support on the underframe. The leaf springs were obviously replaced to passenger type instead of goods wagon type. See below:
Photo off the Internet possibly from the SFS site.
Photo Stefanos Vardaris
Either some were converted to Brake Vans at that type or that happened much later from converting the Excursion carriages to Brake Vans. See below:
Both photos off the Internet possibly from the SFS site.
christo.