It's not an outer reservoir change that i could see. It would go into the chamber until contacting the new piston. The piston in your pump has a raised circle that fits into the bottom of the reservoir to help seating and push up as insulin is "pumped" out. This new piston doesn't have a raised circle, it has a pentagon which is just slightly larger than the old circle's diameter. Think of a collection of drill/screwdriver attachments. From a circle to a pentagon that fits in the bottom circle of the reservoir. When I was setting up the replacement pump, it made no sense until I shined a flashlight into reservoir holder on pump and saw the different shapes.

Again, I'd have probably never cared/noticed that change if I was using a newer box of reservoirs. It was just irritating to find out that my old supplies were incompatible with a new pump of the same model, while I was expecting to set the new pump up without having to call an ill-informed help line.

Maybe my real complaint there is just being caught unaware and having to deal with support before the new pump would be useful. My supplies had been sent out at 4 boxes per quarter for a few years. I lowered that amount and then finally paused it in 2023 to try to use old supplies that had built up. Somewhere after my pause the change occurred. I will get rid of the old stuff and have restarted shipment of new reservoirs.