How common is the use of 'although' as a preposition rather than a conjunction?

Question

Conceding a point can be expressed by means of

– an adverb (however, nevertheless, etc);

– a preposition + the point in noun form (despite this, in spite of this, despite the fact that, etc);

– a conjunction + the point in the form of a clause (although this is the case, while this is the case, etc).

In Cambridge's First Certificate Trainer, Six Practice Tests, I found the following (p. 87, key p. 208) in a multiple choice exercise about a text entitled 'Fingernails growing faster':

[…] the human fingernail now grows about 3.5 mm a month, compared with just 3 mm seven decades ago. Toenail growth, [gap: A although; B despite; C however; D nevertheless] only about 2 mm per month, was also up on the figure obtained in a similar survey done 70 years ago.

And the answer the key gives is 'A although'. So here, 'although' is used as a kind of preposition! How common is that?

Answer 1

I suppose that it is a case of leaving out part of a sentence or clause, as in certain relative clauses:

Albert Einstein, who is the most famous physicist of the XXth century, was born in Ulm.

can be reduced to

Albert Einstein, the most famous physicist of the XXth century, was born in Ulm.

leaving out 'who was'. Similarly,

Toenail growth, although it was only about 2mm per month, was also up on the figure obtained in a similar survey done 70 years ago.

can be reduced to

Toenail growth, although only about 2 mm per month, was also up on the figure obtained in a similar survey done 70 years ago.

leaving out 'it was'.

The absence of a subject and a verb make 'although' seem to be a preposition here but it is definitely not. Another case of sentence simplification which complicates the task of whoever wants to understand and learn the language… !