Let's try this:
"The Model nomenclature that Napier's used originally prior to the 6 cylinder cars, was a letter followed sometimes by a number but from the 6 cylinder used type ( T ) numbers, not in chronological order to describe the different models."
So, using a bit of intuition and wild-assed guessing, I figger the T26 30hp would be a good candidate for that year range (1906-07).