Chosen Solution
My watch is A1553 with 38mm case. The back cover on the battery fell out and I need a replacement. Otherwise, the watch has been working fine but no longer chargeable.
While the external cover @oldturkey03 has pointed you to will be needed, there’s a bit more! All I see is the bottom side of the S1 SIP unit you appear to be missing the charging coil unit and the photo sensor unit. You can see a full teardown here Apple Watch Teardown
