Prototype

Prototype units, are what ODM mainly used for testing the device before the final unit can be released to market.

Generic IMEI
Devies made by FIH Mobile usually will not have IMEI written properly. When FIH Mobile calibrate NV, a generic placeholder IMEI will be used - 004400152020002. It's confirmed being used on many devices.

Placeholder Logo
Before the final unit got released, ODM will usually use some placeholder text to keep the device as secret. "BRAND" text is mainly used, and for prototypes made by FIH Mobile, the logotype for respective OEM will be used directly to write the logo. More variants including "KRAND", "DRAND" exists.

Also, on Nokia 8.1 and Nokia 9 PureView prototype, placeholder text "idaradono" exists.

Not-blown fuse
To ensure Secure Boot for checking bootloader partition signature working, retail units have a fuse inside the SoC blown out of factory. Prototype units however, the fuse is not blown for development purposes.

On known prototype units, simply flash retail firmware will not blow the fuse, but there's one single command under Fastboot mode can blow the fuse.

Blowing the fuse is not reversable unless you replace the SoC.

Different Hardware Variant
Most of early prototype units have different hardware structure to final retail unit, mainly on the PCB layout and components (e.g. screen) FPC port.

For example, earliest Nokia 9 PureView prototype has a fingerprint sensor on the back.