The X - ray image of Berth's left hand, complete with visible

The X - ray image of Berth's left hand, complete with visible bones and wedding ring, was probably not the first X-ray image, but is first to be made deliberately.

1. but is first to be made deliberately .
2. but it may well be the first to be made deliberately .
3. on the contrary, it was the first to be made deliberately.
4. but it could be the first to be made deliberately.

'The first' makes it a noun, while in the question the 'first' is erroneously used as an adverb.