I have to correct I.J. here, the two Webleys are not the only power variable models made. I have an adjustable power model and also know of others who own various models with variable output.
Back in the day a few slant face receivers were produced with an extended valve which could be accessed without removing the stock. I believe that while a few stayed in the country, most were exported. I have seen a thumhole stocked Eliminator in .177 with this modification.