This is from TR 320-35 (Dated 1925)
"The two trasversing stops slide around the outer edge of the round brass flange on the top of the socket and can be set at any desired position by tightening the traversing stop screws (15H). The traversing stop pin (15B) can be moved up or down by means of its handle (15C). When pushed up it clears the traversing stops, but when down it comes in contact with them. When the traversing stops are properly set for the flanks of a target, it serves as a convenient method of regulating the limits of the traversing fire."
15H & 15B are not shown. You have to look through Machine Gun Service Regulations (Section 22), the Handbook of the browning machine gun, and the weapons mount book to figure it all out.
You can make out the (traversing stop pin (15C)) handle at the back of the pintle
These are the traversing stop slides. Its kinda hard to tell what the other parts of the stops are called ,because the manuals I have lack a complete break down drawing.
15F Traversing stop top shoe
15G Traversing stop bottom shoe