The door intercom that you are speaking about is most likely from either Nutone (www.nutone.com) or M&S Systems (www.mssystems.com). I cannot remember or find the published sized of the door intercom station for either company. But both companys offer the brass door speakers. You have to stay with the same brand for the system to work. Depending on how old your system is some of the model numbers are on the master either behind the doors or on the corners. If you cannot locate the master the best bet is to call them direct and describe the system to them.