Thank you for showing this, Roy.
Sowerby patterns 1261 and 1261¾ are shown side-by-side on page 4 of pattern book IX (1882). The only differences between the two patterns seem to be that 1261¾ has a slightly more flared top and a slightly more bulbous body (most obvious at the bottom of the handle loop). Your photo would, therefore, appear to be of 1261¾ rather than 1261.
Photos of the 2 versions are shown on Volume 3 of the Thistlewood Sowerby CD-ROM (1261 in turquoise vitro-porcelain and 1261¾ in blue marbled glass) but I think that, apart from yours, those are the only other photos of the pattern that I can recall seeing - so it would seem to be an uncommon pattern (especially so in green marbled glass).
Cottle (page 100) gives the corresponding Sowerby RD for pattern number 1261¾ as 314281, registered 18 September 1877 [-Parcel 7], and I presume that the lozenge on your vase in in accord with this. I also presume that a pattern 1261 vase would bear the same lozenge and be from the same registered design.
Just for completeness, could you tell me the size of the vase, please.
Fred.