Cal. SZ04 is an “in-house” movement produced by Sinn based on the ETA Cal. 6498-1. It moves the hour hand to a subdial at 12:00, creating a regulator design on the “hunter” watch movement and adds a hacking seconds device.
Sinn introduced Cal. SZ04 in 2006 for the 6100 “Classic” line, which continued through 2024 and appears to be quietly retired in 2025.
Note that this movement is based on the 18,000 A/h ETA 6498-1, not the 21,600 A/h ETA 6498-2 that came out a year earlier.