Example
"Ms. Desai bought shares in FTC International, manufacturer of a wide range of smartphone accessories."
From Santa TOEIC (WEB SERVICE)
Hypothesis
"which is a" is omitted before "manufacturer".
This is a non-restrictive clause.
Ms. Desai bought shares in FTC International, [manufacturer of a wide range of smartphone accessories].
I wouldn't say this is a non-restrictive construction with the relative phrase omitted.
The bracketed element is non-restrictive, but it's an ascriptive NP supplement, more specifically a 'bare NP', i.e. one with no determiner.
It would be possible to add a determiner: a manufacturer of a wide range of smartphone accessories.