Wolfgang, remember that virtually all of the Gibbs ledgers were destroyed when the Germans bombed Bristol.
Mr. Crudgington will be able to tell you when it was made but maybe not much more.
BTW, those Selous type plates look to be very well done. I am not sure if Gibbs offered anything like that on a double as they did on the Farqharsons but the forming of the steel on your gun is very similar to those on a Farq though they do not meet up to the action body.