Its definatly a Gilbert Observatory not a Sessions. Sessions never made a head piece with that design. I'm puzzled by the letter "A" on the lower glass. Never seen that before on a Observatory or a "Store Regulator" before. Most of the time it is the Ansonia trademark that sometimes shows up on Ansoinia School house style clocks.
You have a very nice Clock
'Regulator A' or 'Regulator B' by: daniebrady@gmail.com
I have a newly acquired clock made by La Cloche which says 'Regulator A' on the glass. But after much searching for a description it seems like it's a better fit for a 'Regulator B' and the reason is that the 'A' model is supposed to have through the face time adjustment whereas the 'B' model is set by a nut below the pendulum. Mine doesn't have through the face adjustment so why isn't it labeled 'Regulator B'?