The registration lozenge appears to be a ‘mirror’ image, Paul, because you are looking at it from the wrong side of the glass. It is not actually a true mirror image, simply rotated around a vertical axis, and if you rotate it around the vertical axis again (with the III at the top and the 6 at the bottom) – hey presto! - it reads quite correctly (from the 12 o'clock position moving clockwise) B (year - 1958) - 21 (day) - 6 (parcel number) - A (month - December).
Now here is a pressed glass plate, 16.5cm diameter, by Angus & Greener in the same RD 117501 pattern (also bearing a raised lozenge for 21 December 1858 – Parcel 6).
I am struck by the similarity of this Angus & Greener design to a design (for a pattern) registered by Peter Robinson & Edward Bolton of Warrington on 23 June 1856 - Parcel 2 (L - 23 - 2 - M) previously discussed you, Paul, and Roy at in the opening posts of a GMB thread athttp://www.glassmessages.com/index.php/topic,40114.msg263288.html#msg263288
If I hadn't have known they were different registered designs, from a cursory examination, I would have assumed that the Angus & Greener pieces and the Robinson & Bolton pieces were from the same registration, (though close scrutiny reveals that the Angus & Greener design is two rows of unadorned ovals, whereas the bottom row of ‘ovals’ on the Robinson & Bolton pieces are more like squashed diamonds with bevelled edges). I suppose this is where the registration of minor detailed differences in a pattern gave legal protection against accusations of design plagiarism.
A few additional facts about Angus & Greener that you may find interesting:
Henry Greener was born in Sunderland (already an established glass-making centre) in 1820. At the age of 12 he was apprenticed to John Price, a Gateshead glass manufacturer. He was evidently successful, for at the age of 19 he was promoted to travelling salesman. He subsequently accepted a similar appointment under John Sowerby at the Ellison Glass Works and stayed in Gateshead for some years. In 1858 (the year of his first registered design) he returned to Sunderland, and in partnership with John Angus, a former glass merchant, he took over the Wear Flint Glass works. The partnership lasted for almost 18 years until the death of James Angus, after which Henry Greener moved, and built a new large glass works in Sunderland, of which he was proprietor.