Not sure, but the smooth barrel (before the addition of a slot for stops) with the threads on the outside like that make me think that this is one of a set of lenses that were used to get multiple exposures on one plate. One example of this kind of a camera is here. I have one and have seen quite a few sets since I got mine. If so, it's probable that the lens would have been mounted with the threaded end forward (away from the ground glass). Does the barrel have a number 4 or a number 9 on it? The number stands for the number of exposures done at one time on a plate. Mine will cover 6x9, but only a bit more than that.