Paul — an important discovery, which answers a question I have had about the Stonier business for a long time.
Obviously Stonier's salesmen pushed Stuart's patterns to maximise revenue for the combined family business, but what happened when a client of Stonier's, usually a shipping line, wanted non-Stuart Stourbridge glass? Your discovery shows that the order came from Stonier's through to Stuart's as usual, who then ordered the glass, obviously specifying that the glass must be unmarked, from their Stourbridge competitor, probably at a very attractive discount. Upon its delivery to Stuart, the glass was marked with whichever Stonier mark was available and then shipped to Stonier's. The competitor, in this case Thomas Webb, was unlikely to have been told where the glass ended up.
This sort of business between glassworks probably took place fairly regularly, but little of the ephemeral evidence for it survives. It is known to have taken place between the Manchester glassworks, who even filled gaps in their trade catalogues with their local competitors' products, but this is the first example I've come across for the Stourbridge glassworks.
Sorry about the two year delay — I've only just seen this post!
ps — a similar scenario might also apply to Fortnum & Mason's glass, known marked: "MADE IN / F & M / ENGLAND", but I don't think we've even established a preferred Stourbridge glassworks yet for the few examples known.