Another possibility is that this gun started out in life as a high end, engraved model from a London gunmaker. The side lever decocking knob alone is usually an indication that this was a high quality model. To try and pass this off as an Austrian Contriner, the only addition needed would be the "in Wein"
Of course, this very much like our well known very suspicious Austrian Military models in which the only difference between a plain "civilian" model and a "military" model is the addition of a couple number and a "G". Instantly, a plain Jane no name English copy of the Girandoni is transformed into an incredibly rare and valuable historic air gun.