The Ex HP Laptop HDD was a UEFI GPT Drive. Four Partitions, only Three of which were used.
I converted it to a MBR Drive, so I expect the Boot Table will be placed on the 1st Reserved Partition - 277MB Unallocated.
So you converted UEFI bootable drive from GPT to MBR and expect it to boot?
No. It doesn't work like that.
Bootloader has to be completely recreated after this.
Why did you convert it at all?
What software did you use for the conversion?
Also there's no guarantee, you will be able to boot it to windows on another computer.
On another computer it's recommended to reinstall windows.
If you try to avoid reinstall, then you'll get several issues:
boot mode compatibility - system doesn't boot,
drivers incompatibilities - bsods, crashes, bad performance,
windows activation isssues - can not activate windows.
Your first issue is improperly converted drive, that has been made unbootable by the conversion.
You'll get those other issues (mentioned above) after that too.