IInterestingly, even after booting with Command-Shift-Option-R, the Internet Recovery installer insists on installing Mojave, which came on this refurb computer but was not the original macOS for this 2017 iMac — what originally shipped with this model was macOS 10.12.4 (16F2073).
What I'm wrestling with is the fact that the iMac would not boot from a Recovery partition I created using Carbon Copy Cloner while booted from macOS 10.12.6. (It does boot from a macOS 10.14.5 recovery partition.)
I reinstalled macOS Sierra from a bootable Sierra installer flash drive. This reinstalled macOS 10.12.6 on the iMac's internal drive, and it created a macOS Sierra Recovery partition that