Thanks for talking through this with me. It’s funny I search ETA 2447 all over but it wasn’t until I search “ETA 2447 diameter” that info about the stem appeared.
I’ve ordered a 2360 stem - hopefully problem solved.
I disassembled a Seiko 5139A today and discovered oil—and it is green. The entire movement down to the bare plate was covered in oil. No wonder the amplitude was almost nothing and the Balance was straining to move.