This one. Was remembering it fondly tonight. I haven't had a Zipper since it got stolen in 1979, but I remember the day I bought it vividly. I was 17 and wanted a cool pedal for bass, so I tried the Mu-Tron, the MXR, and the Zipper, and liked the MXR but liked the Zipper the most. Did not particularly care for the Mu-Tron, quite honestly. I like it when Bootsy uses it, but I didn't dig it. But I loved that Zipper and still haven't found an EF I like better to this day. Or maybe I have and I just didn't know it...1979 was quite a few years ago In searching about it, I came across a sale thread on here that said it was the same basic circuit as the Mu-Tron III. Now I couldn't tell you why I had such a strong preference for the Zipper, but I just heard a demo of one that grygyx from bassfuzz.com did a couple years and it all came rushing back to me. But I haven't touched either one in years, and the closest I've got to a Mu-Tron in recent memory then is the Ibanez AF-9, which did sound like how I remembered the Mu-Tron. So if you're more recently familiar with both than I am (I'm hoping at least a couple folks have/had both), how similar are they? And why might I have had a strong preference for one or the other? Wish I could just go buy one of each and sit with them for a few days but that ain't gonna happen till I hit Lotto And I'm not about to try and add a pedal the size of the original Zipper to my pedalboard. Would not mind a clone in a 1590b, though. Do they exist? I haven't seen them if they do.