I like these, they're attractive............. do they double up as drinking glasses as well as egg cups? You don't give any indication of size.
On your side of the pond, the expression 'old flint glass' probably carries more specific meaning, used in respect of pressed and cut glass, than it does in Europe, where crushing flints for the silica hasn't been a working practice for a long time. The expression has also become a misnomer for lead glass, and often simply a name for clear glass, since that was the main end result of the use of Venetian pebbles (flints).
There's much interesting reading in Hudson Moore's 'Old Glass European and American', where he speaks of the very early days of B. & S. - where it seems some of the main reasons the company was located at Sandwich was the availability of standing timber for the furnaces (prior to the use of coal) - the important point for us though was the availability of sand. It seems that despite the use of sand exclusively for their glass during the entire 1820s to 1880s period, they still spoke of their products as being 'flint glass' - presumably lead went into most of their mix.
The reason for all this waffle, is that just possibly there may be some folk who actually interpret the expression literally and think that the glass is made from flints.
Coming back to the egg cups, as already commented, generally speaking it's not easy to find reliable attributions for these machine needle acid etched patterns - the quantity made, on both sides of the pond, between the 1850s and 1930s, was truly prodigious and an exact match is needed to be certain, and floral patterns must have been high on the list of designs which were very common. Coming back to B.& S., and simply as an example - they apparently sent an employee to Europe in the 1860s to study the process, and he returned with an etching machine which the company used.
Of course, if these double ended egg cups are a design known specifically from the States and nowhere else, then that does narrow down the origin - if not then I've no idea as to their place of birth.
In the U.K. it's possible even in 2016, in charity shops and fairs, to find vast quantities of machine acid etched glass - and here in the old country we'd probably refer to such pieces as machine etched rather than 'old flint glass' - just one of the differences depending on where you're standing.
Apologies this is long winded.