STEREOSCOPIC LANDSCAPE LENSES
A. (Andrew) Ross, London circa 1860
Stereoscopic landscape lenses marked "A. Ross, London". Having consecutive serial numbers of 6823 and 6824 suggests they would have been a matched pair. Their simple slip-style lens covers and push/pull focusing are indicative of earlier European landscape lenses.
Per Wikipedia, Andrew Ross began his business in 1830 and by 1840, his lenses were marked "A. Ross". About 1860, following Andrew Ross' death in 1859, the company was run by his son Thomas and was known as Ross & Company. Ross lenses would continue to be marked "Ross & Company" into the early 1880's, and by the 1890's, they were marked "Ross, London".
This timeline, together with serial number ranges as outlined in A Lens Collector's Vade Mecum by Matthew Wilkinson and Colin Glanfield, would suggest these lenses to be no later than 1860, placing them within the early wet plate era.