Just putting my tuppenceworth in!
Is the silver letter "in" the glass actually engraving, rather than silver in the glass?
I recently found a piece of american "Cambridge" glass, which is pressed glass which has been engraved. There are loads of folk interested in collecting it in america, and there are tons of different frilly patterns engraved. The "silver" letter looks very like the "Cambridge" style of engraving.
However, Webb is an English glassmaker.
Perhaps some americans imported some Webb glass and had it engraved by Cambridge, or in that style, which was fashionable at the time?