If it doesn't say, it's PROBABLY a bay 1, as those were the most common. Look carefully at the photos of the Rolleinar - they may have a roman numeral I, II or III on the front ring where it indicates which Rolleinar it is. A bay 3 Rolleinar 2 would say "Rolleinar 2 III" for example. But they changed this marking over time, so that may not hold true. When in doubt, buy one that is clearly marked in the listing which it is, AND buy one that has a return privilege. Or find one at KEH.