I'm no expert, & there are people out there that have been able to tell me who signed off some of my Webleys & the day it was - but a quick look at the Gordon Bruce book says smooth bore 'Senior's were available pre war, the model were introduced in 1930, with a revision in '35.
It also says 'During the first year of production, before serial no's reached S1800'.
If you can post pics, that would help. If you can't, more details - such as the lettering/patents/Webley address stamped on the sides would.
ATB
Webley Mk3 x2, Falcon & Junior rifles, HW35x2, AirSporter x2, Gold Star, Meteors x2, Diana 25. SMK B19, Webley Senior, Premier, Hurricane x 2, Tempest, Dan Wesson 8", Crosman 3576, Legends PO8.