I have a Wm Moore & Co. hammer gun with a side lever on the left side. It is Marked Wm Moore & Co. London. I have not found any info as to when he signed them like this. It appears to be a early to mid 1870's Gun. Any comments?
I'm assuming your gun has British proof marks? There were many Belgian guns bearing the name "Wm Moore & Co.", taking advantage of a well-known name.
Though a gunmaker as early as 1808 (according to the IGC database), in 1836 the name became William Moore & Co., at 4 Whittall St., Birmingham. In 1837 the business moved to 35 Loveday Street, and it was also the start of the business of Moore & Harris (William Moore and William Harris). In 1847 William Moore and William Patrick Grey started at 78 Edgeware Road, London, as Wm Moore & Grey but William Moore also continued to trade as Wm Moore & Co. from this address. In 1854 William Moore & Co moved to 43 Old Bond Street, and Moore & Grey started to trade from the same address. In 1861 Moore & Harris moved to the Great Western Gun Works at 91 Constitution Hill, Birmingham, until 1864 when William Moore may have died (date uncertain). The 43 Old Bond St. address lasted until 1878. Boothroyd has slightly different dates for the various businesses, but a "Wm Moore & Co." gun like yours would have been made no later than 1878.
Guns bearing his name have a variety of inscriptions. I have a 16-bore pinfire of his made around 1862, with the early Lang-type single-bite rearward under-lever with assisted-opening stud. This gun is simply marked "WM & Co."