OK, here's my 2 cents worth. My 135BC is about a year old and I just figured it was like the larger Holgas, and the switch did nothing. But in looking with back open, I put the bulb switch on, and tripped the shutter, and while it was open flipped the exposure switch back and forth. It appears that a smaller square slides over the aperture but doesn't really restrict it in anyway I can see. I don't see how this would have an effect on the exposure. Check for yourself and see what you think. I haven't heard about any changes made in the camera, but then I haven't really been looking for the info either.
I know that Michelle Bates in her "Plastic Camera" book explains how to take the full size Holgas apart and add your own aperture in one of the squares that slide behind the lens. You can even add a piece of colored gel, like yellow or red, to have a easy filter if you mainly work with B&W. I don't know how easy it would be to take the 135BC apart to do this. Maybe her newer book covers this.
Hope this gives you a little to go on. Good luck.