This is Alfred Bicknell's Sash-Cord Guide, Patent Number 83,685, issued November 3, 1868. I have a set of four, and have seen one other unit for sale (mis-identified as an insulator), but no others: they are rare. The embossing reads "PAT.NOV.3 68" on one side and "NO 2" on the other. Presumably there are several sizes. Dimensions are 1 13/16" (47mm) long and 1 3/16" (31mm) diameter. All are made of clear glass.

These were alternatives to the typical pulley-wheel guides used in older single and double-hung windows which use a sash-weight on a cord as counterweight:

My examples have the square notch (a³ in fig. 3 of the patent), but the glass was introduced into the mold at that point so there is a "pontil" left over which has not been ground off, thus making the slots useless. The Corning Museum of Glass has a photo showing them inserted into pieces of wood, but it's not clear how they are retained in position.