Found Ash's CERTUS thread from 2014
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=246676&page=0&fpart=1&vc=1

From the 1903 catalog, smokeless cartridge list offered for the "New Model" CERTUS:
.256 Mannlicher (6.5x53R)
.303 British
.375 2 1/2"
.450/.400 3 1/4"
.450 3 1/4"
& .450 #2 Nitro (only in single shot)

There was a "Rook, Rabbit, and Gallery" model offered in .220, .250, and .295

I have never heard of a surviving example of a CERTUS in .256 or .375 and only have seen an auction advert for one CERTUS in .303.
It would seem likely that, unless a huge Cogswell & Harrison fan, a sportsman would pick a less "clunky' rifle and select a Mannlicher or Lee Speed for a small bore. The predominant calibre amongst the few known CERTUS rifles today is .450/.400 3 1/4" Nitro.

- Mike