To be sure, we must be cautious about the dating of these bells.
The original collector believes they came from the 6th-7th century because similar or related finds coming from the area can be dated to about this time.
This was verified independently by another antiquities firm but the "footed" style of these bells was in use much earlier as well.
The only publication on the subject that we know of (N. Spear, "A Treasury of Archaeological Bells": Hastings House Pub., New York; 1978), shows examples found at Pompeii, which guarantee that the style was in use at least as early as the 1st century AD (p. 180); another example is attributed to the 3rd-1st centuries BC (p. 183).
So they may be much earlier than Byzantine!
Unfortunately, the book does not say much about later periods, so without doing a lot more research, we have no way of determining the latest date to which the style can be assigned.
They are certainly not later than 8th-9th century.
The simple fact is that without context, the dating of antiquities is educated guesswork, and generally one can only say is how long a given style was used. This is a common difficulty with antiquities other than coins.
But we do consider the 6th-7th century, and therefore Byzantine, to be a best professional guess. They are almost certainly that, or older.
And if they are older, they are still somewhat likely to have remained in use into Byzantine times, since the ancients were not so wasteful as we are.
So we do not hesitate to call them "Byzantine bells".