When were these fasts observed?

Question

The Tur (Orach Chayim 580), quoting the Bahag (Laws of the Ninth of Av), gives a list of days on which to fast. This list is copied or mentioned by some other rishonim (as quoted in the footnote to the Bahag: Eshkol part 2 p. 9, Shibolei Haleket 278, Tanya 62, Seder Rav Amram 34a, part 2 74a (?), Machzor Vitri 271, Kol Bo 63).

Some sources (e.g. Magen Avraham) say the list comes from Megilat Ta'anit, but this appears to be incorrect. Although they are written in the style of Megilat Ta'anit, all the dates are in Hebrew, not Aramaic as Megilat Ta'anit is, and the response of the ge'onim (quoted in the Shibolei Haleket and Tanya) also implies that Megilat Ta'anit is separate from it. And the death of Tanna'im is also mentioned in the list, making the second century the earliest possible date, at least for its final form.

The response from the ge'onim that shows that they didn't know who decreed these fasts. The Beit Yosef says about them that he never saw anyone observing any of the fasts, and expresses surprise that some of them are on Rosh Chodesh.

My question is: Apart from the question to the ge'onim, is there any sign that these fasts were actually observed at some earlier point (not necessarily the entire list, but excluding 17 Tamuz, 9 Av, 3 Tishrei, 10 Tevet)? Are any of the days mentioned as having been a day on which someone fasted in any of the sources, especially before the time of the ge'onim?

I ask because Josephus (Antiquities 14.4.3) mentions a fixed fast in the third month (apparently in Sivan). Some (William Whiston in the footnote there, and this book I found through searching) suggest dates from this list as the fast he mentions. However, if the dates never appears until the ge'onim, it seems unlikely he was referring to them.

Answer 1

I happened to find a similar list of fast days in the Chronology of Ancient Nations (chapter 14) of Al-Biruni. Many of the fast days are given on the authority of a Jew Ya'qub ben Musa al-Niqrisi from Jurjan. This would seem to demonstrate that these fasts were observed by Persian Jews in the 10th century. If we assume the fasting predated the list of fasts, then accounting for the lifetime of the author of Halachot Gedolot would seemingly imply that the fasts were observed at least in the eighth to tenth centuries. The fast days are characterized as "voluntary" (p. 270), which is in sharp contrast with the list given by the rishonim which refers to these fasts as coming "from the Torah."

Another list, found in an inscription in a synagogue in Beth Shean from the fifth-seventh centuries, is described here by Haggai Misgav. It has much fewer dates, but is interesting in that it comes from Israel rather than Babylon, and seems to be earlier than the others. The author of the article judges it to be reflective of an actual practice because of its practicality (it doesn't give reasons for the dates as the other lists or piyyutim do, and because it records more than one fast day per month, which seems more reflective of a practice than of a scheme; the latter reason would also be valid for the list in Halachot Gedolot)

The differences in date between the Tur, Al-Biruni, and the inscription:

             T  AB  I
             ----------
Tishrei      3   3  ?
             5   5  5
             7   7
Marcheshvan  7   6* 5
Kislev       28  8# ?
Tevet            5
             8   8
             9   9
             10  10
Shevat       5   5
             23  23
Adar         7   7
             9   9
             $   13
Nisan        1   1 
             10  10
             26      27
Iyyar        10  10
                     18
                     22
             28  28
Sivan        23  23
             25  25
             27  27  27
Tammuz       17  17  17
                     27
Av           1   1   ?
             9   9   9
             18  28
             25  
Elul         17  7   5

* or the Monday between 8th and 13th
# or the Thursday between the 19th and 25th
$ 13 mentioned in Tur Orach Chayyim 686
? unclear