Have 29" Series 385 Channel Balancerams but the stop that screws into the top of the window is broken so the track just shoots upwards. I haven't a clue what it would even be called and there were no stamping a on the part.
Tom from SWISCO responded:
October 8, 2020
This part is called a top sash guide. Yours in particular looks like a good match for our 18-124P pair. Look these over and let me know if you have any questions.