The gun just looks "belgian" to me. However, you say it has London proofs? Hmmmmm. Two things come to mind. One, it is an outright fake, complete with bogus proof. Two, the Manton brand was carried on in a shop in Calcutta, as I recall, long after there was no Manton or gunmaking in the firm. The Manton name had too much Brand Value (BV) to just die gracefully. Soooo, perhaps it is a gun that passed through their shop much as such "name" guns passed through American hardware stores.