Hi… No, the Machine ID is what you want, the first column of fly m list.
I just tried over here on a small test app of my own, and fly-replay: instance=<machine-id> does work. (It also conveniently auto-started the other Machine, since I had auto_start_machines = true in fly.toml.)
Are you perhaps crossing an app boundary in your case? In that situation, you need an explicit app= knob, last I checked:
https://community.fly.io/t/fly-replay-header-does-not-work-with-instance-id/23852