An early Elgin size 18 -model 1, grade 18, class 1 movement (that I originally thought to be English), with a Swiss Lever escapement but laid out tangentially as an English Lever. It has 11 jewels and an over-sprung cut compensating balance. It winds clockwise from the back and is set from the front by using the key on the "hub" holding the hands. Dated from the serial number to c1873.
The English double backed, triple hinged, case has Birmingham hallmarks for Sterling Silver, 1875 and the makers mark of A.L. Dennison. This is a rare and very early case by Dennison who came to dominate the English volume case making industry and was second to none in the field world wide.
Priestley's "bible" on English watch cases says that Dennison family records show the business started in late 1874 but it also shows that the ALD makers mark was entered on the register (presumably now, not for the first time) on the 20th April 1876. The back lid of the case has the date letter used in 1875 and again in 1900 with the shield cartouche indicating 1875, the Birmingham Assay mark with it is also in a shield for the 1875 date series.
However the lion silver mark on the back lid and the date and silver marks on the dust cover (the assay mark is not required to be repeated) have the cartouche for the 1849 series used up to 1874/5.
So the case was certainly made in the first 12-18 months of the formation of the Dennison company but the probability is that it was made as the hallmark date sequence changed in early 1875 making this a case from the first few months of production.