It appears to be an indicator of upon which models the upgrade can be applied.

Yup.

But the ``car1'' upgrade's chunk contains ``1\n'', which makes intuitive sense, but doesn't match the /proc/empeg_id ID for the Mk1 of 4.

In order to cover various prototypes and halfway-house development units, some of which didn't have empeg_id's at all, "1" means "none, or anything less than 7".

As you can see for yourselves in lib/protocol/upgrader.cpp, CHUNK_UNTARDRIVE0 was never implemented.

Peter