The men in the photo are, from left, Edward Douglas Brown, William Brown, George Brown, Hugh Brown & Samuel Brown. According to Anne, all but Samuel were tailors in Chesterfield. Examination of census records shows that the oldest brothers George and Edward Douglas arrived with their wives in Chesterfield in the mid-1870s, setting up as clothiers/tailors, followed shortly by William, who was apprenticed to George in 1881. The youngest brother Hugh followed in the late 1880s, and by 1891 George had moved to Wingerworth.
Photographers often advertised that they could enlarge photographs "up to life size," but in practice anything larger than a cabinet card (4" x 6" or 107 x 165 mm) is not often seen - at least in my experience. It's nice therefore to receive an image of an example of a larger format. This mount measures 215 x 163 mm (or roughly 8½" x 6½"), while the photographic print itself is 200 x 155 mm.
Men's clothing is always hard to date, as it didn't change with as much frequency as women's fashions did. However, the thick, glossy dark green card used for the mount is typical of that used for both cartes de visite and cabinet cards from the late 1880s and throughout the 1890s. The firm only started advertising itself with the "& Sons" suffix in the late 1880s, so it is safe to assume that it is was not produced before around 1887.
The back of the mount has a label affixed to it, which is to be expected. A studio would be less likely to go to the additional expense of ordering pre-printed mounts of this size if they were not used often. The label states that the firm of "A. Seaman & Sons" had branches in Chesterfield, Matlock Bath, Ilkeston and Alfreton. Although the other three studios were all in existence for lengthy periods, Matlock Bath very rarely gets a mention, at least on the mounts that I have seen. I believe Alfred used to travel there to take portraits on a regular basis at Smedley's Hydro, rather than have a permanent studio there.
However, from the style of the studio furniture and painted backdrop, I suspect that this group portrait was probably taken in the early to mid-1890s, say between 1891 and 1896. Although the elaborately upholstered chairs with tassells and fringes were more commonly used in the late 1870s and 1880s, I know that Seaman continued to use his well into the 1890s because they are a regular feature of other portraits from that decade on my web portfolio of him.