Hi Baz,
The Certus (means 'reliable') was made by the well known English sporting gun maker Cogswell & Harrison. Production numbers were very low with probably less than 500 being produced around 1929-1930 in an attempt to compete with the all conquering Webley design. There is quite a strong possibility that production ceased after Webley threatened legal action for infringement of their famous patent 219872 which is the basis of the Webley overlever cocking pistol design. The Webley patent 219872 does in fact cover the possibility of using the barrel to cock the pistol in the reverse manner to the conventional method but this was rejected as not being a practical proposition. The Certus is one of only three pistols that I know of that is loaded by a tap similar to the BSA air rifle these being the Certus, Abasmajor & a version of the Lincoln Jeffries pistol.
The Certus was Cogswell & Harrison's only venture into the world of air gun design and although well made the relatively delicate mechanism and inelegant appearance provided little threat to the Webley.
The 'Target' version is much rarer than the standard pistol and there is some contemporary advertising to suggest there was a detachable stock although an actual example is yet to surface. The threading is however present on both versions of the pistol, so it would seem a reasonable bet that there is one out there somewhere
The picture seems to indicate that your pistol has been refinished at some point as the original finish on these pistols wasn't all that great and pistols with much original finish remaining are seldom found.
Quirkiest pistol I have come across which usually attract a high price.
Good luck with the sale.
Regards
Brian