Steam vent ???

Could you help us ID this old item or might have a PDF to describe it operation. It’s from an old vapor steam system. It has a ¾” NPT connection x 5-3/4” H & is inscribed with #5 Thermodisk, I-39, Jas. P. Marsh & Co.-Chicago. Not certain if it functioned solely as a vacuum breaker and/or an air vent. This was located in the boiler room atop the dry return before piping dropped into a boiler return trap arrangement.

PS CANNOT copy photo of device at this time. Uncertain how to attach file.

 Thank you, Mike